yeah, I though the announcement was just that portal 2 will be on the ps3 and will support steam, not that steam is going to be on the ps3. the "big" part of it was due to valve previously badmouthing the ps3, and not giving the ps3 ports of their games any attention (the orange box on the ps3, for example, was terrible), making an announcement like this huge, since the ps3 version will be the only one with support for steam.
still, that was a great part of the Sony press conference, Gabe Newell of Valve got some nice lines in there in reference to his previous ps3 bashing: "I'd like to thank Sony for their gracious hospitality, and not repeatedly punching me in the face" "if I seem nervous, its only because Kevin Butler was introduced to me backstage as "VP of sharpening things""