Sure, by leveraging most of the work that Intel already put into the DB2 port. Once Intel helped IBM to rewrite the code to be 64-bit friendly, of course a port to another 64-bit architecture was easy. Let's see how many applications AMD is able to port without Intel's help.
I agree with the idea that DB2 was probably 64 bit clean prior to the AMD64 port. But I wouldn't necessarily credit Intel with that. IBM has its own 64 bit RISC family and AFAIK DB2 has been running on that under AIX for a while.
wbmw, I know you love Intel but they have zero credit for that. DB2 was already ported to 4 other 64-bit architectures - Sparc, Alpha, HP-RISC and Power4 before Intel "helped" IBM to port to Itanium. IBM must be really confused by this Itanium instruction set to need help.
Edit: Intel is just #5 in that 64-bit game, almost the last and very far from first