Valve may be preparing a 64bit version of the Steam client with an update in the Steam Beta Client that was released yesterday.
Their wording in the patch notes certainly suggests that's what they're doing:
Added support for shipping different binaries to 64bit vs 32bit operating systems in Steam self-updater. This support is being added in preparation for future updates.
Considering the client is already 64bit on Mac, it would make sense to bring that to Linux and Windows soon too. With most people now on 64bit, it was only a matter of time before they did this. Going by the Steam Hardware Survey, few people remain on 32bit. Giving their updater the ability to know the difference between systems, would be the first step towards rolling out 64bit to those systems that support it and eventually warn people on 32bit systems when that eventually becomes deprecated for the Steam client.
We already know they're planning an overhaul of certain parts of the client, with various leaks and Valve eventually talking about it. It's entirely possible that this is in preparation for that to happen.