I have installations running on Centos 4.4 x86_64 and they work fine, no problems.
me 3. Works perfectly fine though I do not believe asterisk takes advantage of the 64bit processing system in 1.2.X (I can be wrong this is what my memory is telling me )
I tired it but changed back to 32bit.
I had troubles with the zaptel clock which was not accurate with 64 bit (RedHat EL4). I needed the clock for meetme
i had also problems with 32bit smp and ztdummypll. This was also not accurate. I solved it now by using a singel-processor-kernel.
Now I am running asterisk on a 64bit dualcore-dualprocessor machine with 32bit and single-processor kernel but it is working