If it is a SteamPlay game then you don't need to rebuy. A list of currently supported titles can be found here. Valve have said that all their own games will be SteamPlay compatible, they will be releasing their titles slowly over the coming weeks each Wednesday.
does anyone now what games are been released on each wednesday, just wondering what goodness we get this week
God I really want to get back into some CS:S and DOD:S, spent a worrying amount of time on those games years and years ago.
Shame I never got any good
C'mon Valve!
loved DOD:S only thing that ticked me of about it though was been tked for the sniper rifle made my blood boil
Love the way they call it cross-platform multiplayer week, yet not TF2 or CSS.