Can't fault the Pi for the money, my 12 year old is getting one for Christmas. The really important thing is for it to have a point - I remember doing "computers" at school and thinking it was all very well to turn the screen blue, but there was no point. I only "got it" at Uni when the calculus made my brain melt and I was doing compressor energy recovery simulations in an unholy mixture of Fortran and Excel 3 and absolutely loving it.
The nice thing about the Pi is the ease of physical interfacing, and the fact that there are loads of projects on the web. I've got him a temperature/humidity sensor, and he's going to build a web based sensor for the attic. Also a £5 bluetooth dongle will let him stream music to his boombox (once he installs Squeezebox or similar).