Become a MacRumors Supporter for $50/year with no ads, ability to filter front page stories, and private forums.
Some Apps dont work right with the 64-bit kernel

Some kernel extensions and drivers might not work. Not apps themselves, but apps that install kernel extensions and drivers. Most of them will work (all mine do) so I wouldn't worry about it.
 
As mentioned before, some Mac Pros and xServes have 64 bit enabled by default. Most certianly the recent ones.

XServes (dunno since when) and the 2010 Mac Pros come with 64-bit enabled.

Some Apps dont work right with the 64-bit kernel

the 32-bit kernel can run 64-bit apps, and the 64-bit kernel can run 32-bit, with no problems. they just run it in the corresponding mode.
 
Register on MacRumors! This sidebar will go away, and you'll see fewer ads.