I personally feel that you'd have to recompile the application using a 64-bit compiler (obviously on a 64-bit machine) to get the most of Memcached on a 64-bit platform. This may not be an easy task depending on the code. If it was written with 64-bit portability in mind then it could be a simple recompile. If it hasn't, then you could well be up for quite a bit of patching before getting it to build.. and then you'd have to verify that you haven't broken anything!
I don't think you're overestimating the differences between 32 and 64-bit at all. A common mistake is to assume that the job is a simple recompile when in fact it isn't. There are more portability issues than most people realise. Just because the application builds and you end up with a binary, it doesn't mean that the binary is going to behave as it should. Especially when it may interact with other 32-bit code.
Having said that, it might be worth giving it a spin!
Good luck. Cheers!
@Lars: I recommend reading the question before attempting an answer.
@John Sibly & @DannySmurf: given the nature of Memcached and what it aims to achieve, surely you wouldn't want to run a 32-bit version on a 64-bit machine? If you had a 64-bit capable machine it would make sense to run a 64-bit version to make the most of the features of the hardware.