I personally havent tried HL2 yet, however i will be later tonight. On the other hand, I have tried another game that came out around the same time as HL2 did, Doom 3 within windows xp under bootcamp. Let me tell you, I cranked it to Ultra settings (Highest it'll go), and every advanced option set to full except for i had AA off, and didnt turn on Vsync. The frame rate i achieved was unbelievably good! I ran the time demo, and it said i had 70 fps!
You wanna know what else is strange? A friend of mine who just bought a new vista laptop with a 256mb gfx-card ran the same test, and got half what i did on my SR-MBP with 128mb ghx-card.
All in all, im happy with the frames i got, and on my PC Half Life 2 has always run better than doom 3, so i imagine that HL2 would run flawlessly.