We too are almost TV-free, and I too have been a little confused by the computer thing. My eldest began using the computer at 2.5, as have my middle two kids. Strangely enough my youngest, who turns 2.5 on Monday, isn't interested yet. But I suspect she will be soon.
What concerns me most about the computer, besides its anti-social properties, is just how shallow the learning is. So much of the educational (i.e. "good") stuff on the computer is binary in nature -- there are no shades of gray, and the object is to click the right things so that the right results occur. Illustrations are flat and cartoonish, the colours are bright and primary, the screens change quickly and games are filled with rewards and scorekeeping and canned "Yay, you got it right!" voices that have nothing to do with a real person.To me this sort of exposure promotes simplistic learning where the object is to choose the right rather than the wrong answer. Exploration occurs within an extremely limited range of options. Creativity is limited to choosing the wrong answer on purpose.
In the real rather than the virtual world, life and learning aren't like that. When you do something, the results are sometimes unpredictable. Things aren't just black and white but filled with shades of gray. Sometimes you can't really tell that A goes with B, and sometimes, for whatever reasons, A goes with C instead. That rich unpredictability and infinite world of possibilities is something I don't want squeezed out of my kids' learning lives.
I tried to get around this by choosing creativity-driven software for my kids. For a long while KidPix was the main program the kids had. We bought it when my eldest was 2 1/2 (and upgraded 3 years ago or so) and the kids, now 11, 8 and 6, still rate it in their top five favourite programs - that's nine years straight! We have a lot of simulation-style games where there are so many variables that there is the appearance of a shades-of-gray virtual reality. The other big thing that gives me a degree of comfort is that they use the computer socially most of the time, two or three of them playing co-operatively, discussing what they want to do, generating ideas. I also see some positive use of the computer for learning and creating: my eldest two use word processors for creative writing and study music theory using a computer-based program much more avidly than they did the pencil-and-paper program they were originally assigned, and I really like the use that the Rosetta Stone second-language software has got around here.
My approach with TV has been to not restrict viewing time, but to model sensible use, provide interesting alternatives and encourage my kids to do the same, and the result has been an almost TV-free existence where the TV often doesn't come on from one week to the next. I'd hoped the same would be true with the computer. But the modelling of good computer-use habits by the parental units is much more challenging

, and I confess my kids are computer addicts

.
Recently they decided to try using
WatchDogPC to set themselves some limits. One kid chose 30 minutes on, then at least a 30 minute break. One chose a 3-hour per day limit. Another had a combination approach. It lasted 3 days and then they began circumventing the controls to de-limit their play.

(Not that I couldn't use the software to unilaterally prevent them from doing that, but I'm trying to be a non-coercive parent -- and I keep thinking that this is a prime opportunity for them to learn to set their own limits.)
So... the computer is a big problem here, and it began with some innocent mousing about at age 2.5. I can't predict the same for you; clearly others have had different experiences, but this is ours. Having said all this, my kids are quite physically active outdoors most days, they are terrific, well-behaved, empathic kids, they're accomplished musicians and have a myriad of other talents -- so they haven't been "ruined" by all this computer time. But I regret the slide down the slippery slope into computer addiction.
Miranda
(whose 8yo made $35 busking with his viola at the market yesterday and immediately spent it on "the Titans" upgrade for "Age of Mythology")