Noticed it too, already have a normal copy though, don't feel like spending more just to have it on Steam. The 50% off on the Rockstar Collection looks alot more appealing to me; all of R*'s PC games for 42,50 for a week.
Seems fair, I was pondering whether or not to get it as I've been trying to pirate it without much luck to just see how well it'll run, as I don't want to spend money on it just for it not to work well! Already played through it on the 360.
At least this way I'd only miss out on a fiver. :7
Shall download later for sure, my 25Mbps connection is being throttled to 2Mbps atm < damn BT!