Considering that you can still buy DDR ram, I would say a lot longer then that - it will be mid to late 2008 minimum until DDR3 starts to hit the upper mainstream due to current cost.
From Umart (Australia)
DDR3
Asus 2GB(2x1G) DDR3 1333MHz 240pin - Internet Price: $459.00 Corsair TWIN3X2048-1333C9DHX 2GB (2x XMS3 1GB) PC-10600 (1333MHz) DDR3 RAM - Internet Price: $599.00 (
DDR2
DDR2 8G PC2-800 6400Mhz DDR2 Quad Channel(4x2G)4-4-4-12 EVO with MTCD Heatsink GeiL - Internet Price: $625.00
DDR2 2Gig(2x1G) PC6400 800Mhz Dual Channel Geil4-4-4-12 -Internet Price: $105.00