At the end of the year Left 4 Dead will still be above Killzone 2 in sales.
Currently:
Left 4 Dead - 2.03 Million
Killzone 2 - 1.24 Million
If by the end of the year Left 4 Dead has higher sales than Killzone 2 I win, if Killzone 2 has higher sales than Left 4 Dead you win :)
Edit - Oh yeah the usual prize, control of losers signature for a month.