According to the DPI blog, Domino runs on the following 64-bit platforms as a 32-bit application:
* Linux on zSeries
* Linux x86_64 (Technote #1231920)
* Windows 2003 x64 Edition (requires 7.0.1 and higher)
But then they state that Domino is actually a 64-bit application as it runs on i5/OS. However, from the first comment it appears that this is going to be coming in the future. My understanding is that Domino is only a 32-bit application in its current iterations, but will be ported to 64-bit in Domino 8(.0.1) (but will also have a 32-bit port). So this DPI blog post was somewhat vague until the 2nd paragraph of the first comment (unless I've missed something) when we read "When 64-bit support for Domino becomes available, Domino for i5/OS will also change to use the i5/OS 64-bit pointer and teraspace support."
Regardless, the post also states that "i5/OS only runs on 64-bit hardware and has been enabled for 64-bit for a long time. i5/OS also provides 128-bit pointer capability." Yes, you read that correctly - i5/OS runs with 128-bit pointers! If you read the quote below, you will see that when the AS/400 (System i) architects were designing the RISC processor-based system, they were going to use a full 96-bit address space back in 1990!! but actually decided to scale it back when they went the PowerPC route! Some of these other Operating Systems (Windows and *NIX) have come to the game rather late.
The best quote from my posts is from my second link above which I'll quote again here. This is from an old book I have here at the office titled Inside the AS/400 by Frank G. Soltis. I have the edition from way back in 1997:
"The AS/400 is the first and only system in the world to have completed the transition to 64-bit computing.... The challenge for the computer industry is how to enable existing software to take advantage of the new hardware designs. The AS/400 is the only system to have met this software challenge - all AS/400 applications are fully 64-bit enabled. No one else is even close.... In contrast, the AS/400's technology-independent architecture protects the system's customers and ISVs from such disruptions as they move to the new 64-bit RISC processors. Existing application software can immediately use the full capabilities of the new hardware." (Inside the AS/400, pp. 1-2)Then on page 206 we find this:
"Our original RISC processor that we started to design in 1990, which we called C-RISC (the C was for commercial), had a 96-bit address. We had the room for this large address in the pointers and decided to go all the way. When we decided to use the PowerPC architecture in 1991, we scaled the address back to 64 bits."