I've never done this -- I'm very
aware of the issue, and in fact I wrote a piece for Linux Magazine
on 64-bit computing back in 2004. I have, though, seen posts from people who haven't realized they have 64-bit hardware. Today, the vast majority of new desktop and laptop PCs have 64-bit x86-64 CPUs. I haven't checked, but I expect that even netbook and tablet computers generally use such CPUs, although some use more exotic designs, such as ARM.
Note that running 64-bit code on an x86-64 CPU is likely to produce a modest speed improvement, at best. IIRC, the typical speed boost is in the range of 10-20%, although some programs don't even see that. Also, 64-bit code is a bit bigger than 32-bit code, so if you have little memory, you may be better off running a 32-bit version of your OS.
One more point: Most computers sold today use UEFI
firmware, rather than the older BIOS
firmware. This has an important implication with respect to running a 32- vs. a 64-bit OS, because a 64-bit UEFI PC is almost certain to use a 64-bit UEFI implementation, which in turn means that the UEFI features will only be available if you boot a 64-bit OS. If you boot a 32-bit version of Linux on a 64-bit UEFI PC, you won't be able to access the UEFI runtime services. At the moment, the biggest implication of this limitation is that you won't be able to use the efibootmgr utility, which Linux uses to adjust boot loader settings. This isn't a huge
deal right now, but you could end up losing out on more features in the future if you run a 32-bit OS on a 64-bit UEFI PC. (Note that some early Intel-based Macs used 64-bit CPUs with 32-bit EFI implementations, so they're an important exception to this rule.)