I think the S2 is a great camera to learn on. It really has a lot to show for it. However, I recently got a hold of a D20 (not mine but it's in my possession for some time) and I have to tell you I feel like a REAL man now!!! lol
I took 1 picture with that thing and immediately realized how much better the pictures quality was. When I have it on burst mode, I can't even snap a single picture because it's so fast I always get 2 or 3!! Even with flash, the thing bursts at nearly 2-3 fps.
There is so much to control on this camera, that I feel it's way too much for a single man to handle!!! lol
Seriously, though...A dSLR is a very nice invenstment. I would go that route if:
A) I had the money
B) I felt it was as "useable" as my S2
My S2 is so much smaller. So much cheaper. And it does things the dSLR family can't, such as video! Articulating LCD...yadda yadda...
You have to decide what you want, my friend!
Enjoy either way.