I heard that XP will have support for 64bit memory addressing. If not XP, I believe microsoft said they will support it somewhere down the line.
I agree with Madrat, they are going after different markets. Itanium is going for the high end server market, while AMD is going for the low end server/workstation market. In other words, Intel is going after Sun, and AMD is going after the Xeon. So really AMD is not going to lose anything, If AMD succeeds, Intel will lose some marketshare of their Xeon.