SORRY FOR THE LONG POST - BUT IF YOU HAVE NECK PAIN - READ THIS.
I have had neck/left arm problems too over the last six months and have found it due to bad posture both while using a computer and also at work (dentistry). I have researched it quite a bit because my job depends on being able to hold a sitting position comfortably for hours at a time.
Pathology - why it hurts
The neck/arm pain comes from overworking of the muscles in the back of the neck and shoulder blades with continuous and unvaried holding of a forward tilted head position. The muscles become fatigued and get small regions of permanent contraction ('knots', 'trigger points') within them. These sore points are hypoxic (not enough oxygen) because their blood supply has been clamped shut by the permanent tense contraction, and they radiate pain throughout the muscle. The
neck joint capsules and ligaments also become strained and inflamed leading to long lasting, perhaps permanent damge, even arthritis. Headaches/migraines can also result.
Solution 1 - posture and exercises - advised by my physiotherapist - I was surprised - this really, really works!!
a) Posture - Hold your head almost as far back as it can go - this means your head is above your body, not in front of it and the back of neck muscles don't have to work so hard. Your head weight (5-7 kg, some bigger than others!;-)) is then supported straight down onto your spine. It takes quite some effort to hold this position all day (and it looks a little odd), but it is really important, and really makes a difference. Also you need to arch your back, shoulders back, to give both your lower back, and also your neck the concave shapes (lordosis) they are supposed to have. Less trapped nerves / slipped disks.
b) Exercises - do in this order - really effective
i) Hold your head back in the position described in a) above and slowly turn your head left and right about 20 times, like you mean to indicate "No"
ii) Holding the same retruded head position, tilt your head sideways directly to the left about 10 times, then to the right about 10 times. i.e. the top of your head tilts off the vertical axis to the side. You'll feel things creaking, perhaps painful. The more painful, the worse your necks condition - as you improve over days the pain will reduce and fianlly perhaps go.
iii) Rotate your head sideways in the horizontal plane - like you are looking left or right, but as far as it will turn. Even get your hands to rotate your head past where the muscles can take it (but don't break anything!). This also might hurt if you neck is bad, but will become pain free as you improve.
iv) Then roll your head around in a big orbit to get maximum stretching in all positions.
These exercises are really amazingly effective - I could not believe it after having been a bit skeptical of how a physiotherapist could really help without drugs/surgery. Bear in mind the physio helps just by teaching you to do the exercises perhaps 5-7 times per day initially, less when the problem is under control. Their massage, heat packs etc in their rooms is not that effective. It has to be your change in posture, and in regular exercising over days that makes for an improvement. I got much relieve in just four or five days.
Solution 2 - Hardware
a) The standing desk is a great solution which I have not yet got but it has been condoned in Joel Spolsky's blog as he decided how to refit his developer's office. It works because as soon as you stand, the lower concave shape (lordosis) on the lower spine is restored. The upper spine/head position also then automatically improves too.
b) While standing is a help, it does, as Chris mentions, put a great strain on your legs and perhaps lower back. I think from first principles that the best solution would be a reclining chair with tilted table to hold a laptop or keyboard almost vertical, with arm rests to support your arms up to the keyboard. This 'NASA astronaut' position, sometimes termed the zero gravity position by some recliner chair manufacturers, leaves the whole body's musculature relaxed. There are plenty of recliner chairs for sale, but at a hefty price $1000 - $2,500, but not with arm rests to a clamped laptop/keyboard. I have a rough design, and would like to make such a reclining chair/workstation, but have not the time so far.
It would still be important to move from this reclined position regularly though, perhaps every 20-30 min. No matter how relaxed the muscles are, they can not function well without regular movement. Perhaps a time clock alarm could be built into the chair to remind you to get up and have a walk around and do the stretch esxercises.
What do you guys think? The exercises have really helped me. I am now out of pain and my career has been saved - it could have stopped me from doing dentistry which is such a great and satisfying thing to do. The neck physio info comes from a book by Rioobin McKenzie I got from my physiotherapist. See www.mckenziemdt.org
PS another neck tip is to place a rolled up towel under your neck at night to sleep - it maintains that all important neck concave curvature (lordosis). Pillows which are bulky in the middle tilt your head forward which is bad. The bottom edge of the pillow needs to be bulky under your neck, with a hollow in the middle for your head to sit in. A solid foam pillow can't be shaped properly so a feather pillow is best, or cut up the foam into small pieces so it can be distributed as required under the neck. Some people put the rolled up towel in the bottom edge of the pillow case to hold it there.