Computers are bad for your posture (duh!), we all tend to slump after sitting at them for a while. Set your Outlook or equivalent program to remind you every hour or so to sit up straighter.
Laptops are worse for your posture, since the screen and keyboard are so close together, so you’re forced into a more “head forward” posture. They’re great for short trips when you need to get work done. Try separating the screen and keyboard by using a wireless keyboard and put the laptop up on books for longer use.
iPads/Tablets are HORRIBLE for your posture, since the screen and keyboard are the SAME THING. There is NO good way to use an I-Pad/tablet pc. Minimise use of tablet computers and phones to quick checks of emails, Facebook, web searches or suffer the painful consequences.

***Update*** Contrary to EVERYTHING we physiotherapists have been telling people for decades, recent research has compared thousands of people with varying postural preferences, and found NO correlation between any particular posture and the development of pain or injury. This indicates that there is no ideal posture, rather that the body doesn’t like any single posture for too long a time period. It is lack of movement that typically results in tissues becoming acidic, perhaps a bit inflamed, but that the bulk of the pain developed in “postural overuse” syndromes (like those experienced by some office workers / drivers) was probably more likely due to contextual factors like stress, anxiety, pressure, deadlines, social situation etc. These negative psychosocial factors are sufficient to trigger a protection response in the brain, and thus it produces pain to cause a behavioural change (move, relax, change jobs). So, sit however you like, just don’t sit there too long. If you’re getting sore, move.