Just installed the 10.6.5 update on my machine and all seems to be working well. All the OpenGL screensavers work super smoothly, there's a shadow on my mouse cursor, and I loaded up a heavy 3D graphics scene which seemed just as agile in the viewport as it always has been so I'm assuming everything is OK.
I don't have Steam installed on my test OS X install but once I've updated my main installation I'll test Portal on it.....I'm pretty sure it should be fine though.
I'm pretty sure that if you have a reference card, there shouldn't be issues but if you had to do some ROM editing / hacking to get your card to work initially, then you may have issues that require the help of netkas / Rominator (on netkas's forums) or Cindori.
Hope this helps.