Yup, slipstreaming has always been an integral part of GT racing. Back then I actually discovered it by accident. I was doing the Megaspeed Cup in GT1 and usually I just blow everyone away with my GTO, but for that one time I used the Viper GTS-R and I'm not in the lead for once. So I tucked in behind another car (undeliberately), and all of a sudden I got a massive speed boost! It's like I suddenly found a hidden nitrous button or something. So I researched it on the net, and found out it's called slipstreaming...
Since then I've been using it to outclass other more powerful cars in the straights. But in GT2, like Parnelli said, loss of downforce due to slipstreaming can be deadly in corners (Trial Mt. and Rome Circuit comes to mind). Most of the time the loss is very quick, you don't have time to react. Next thing you know you're heading for the grass/wall. Driving behind another car in GT2 is a skill in itself. I usually slipstream until before the braking area, then I pull out to regain some downforce and braking. Timing is crucial, and even then it still doesn't always work, but with practice you can become better.
I also like how in GT games the effect is not exaggerated visually. When you look at the later NFS games....oh my god....when you draft it looks like you're breaking the sound barrier (especially in carbon). Those of you who played NFS will know what I mean.