Just found Out. Microsoft actually paid off Gabe to delay Half-Life2 so that Halo PC would sell better!
I'm surprised this is the first person to mention this. After reading how buddy-buddy Valve and M$ were at ATI's Expo, Valve using M$ to corroborate their DX9 benchmark results, and then noticing Halo had the exact same release date as H-L2 (originally) had, this sure jumped out at me.
Granted, this is on the 'conspiracy theory' side. I know it wouldn't be the sole reason behind the delay - I think Steam issues are the likely candidate - but I could see M$ asking ol' Valve for a favor. Hell, Valve sold out to ATI. Why not M$ too.