The Ubuntu 12.04 LTS 32-bit release will obviously still be available for older hardware that doesn't support Intel/AMD 64-bit, but the "recommended" version of Ubuntu Linux that will be the default on Ubuntu.com, when ordering Ubuntu CDs, and for Ubuntu USB drives bought from the Ubuntu Store is looking to be 64-bit. Steve Langasek resurrected this 64-bit discussion on the ubuntu-devel list on Monday.
With the Ubuntu multi-arch situation being cleared up, most applications and plug-ins now working fine on 64-bit (such as Adobe Flash and Java), and 64-bit being usually much faster than 32-bit, there really is no big reason not to move to the x86_64 version. Using 64-bit software can lead to higher memory usage, but that's about the only potential downside. In response to Langasek's mailing list message, some Ubuntu developers have been overly concerned about 32-bit netbooks being out there still and for systems that only have 2GB of RAM where using x86_64 software may lead to running low on memory and thus increased SWAP usage. However, for a far majority of users, they're much better off in a 64-bit world. Ubuntu shouldn't be caring about 32-bit so much in 2012. Ubuntu 32-bit will continue to be supported for the 12.04 LTS cycle and likely for all future releases in the foreseeable future, but it's time to make the default switch to 64-bit.
For those interested earlier in the Precise cycle I did publish revise benchmarks of Ubuntu 12.04 32-bit vs. 32-bit PAE vs. 64-bit. Ubuntu 64-bit is generally the hands-down winner across all x86_64-capable CPUs. In order to have the best of both worlds (64-bit performance but with 32-bit memory use), there is the Linux x32 initiative, but that still is being worked on. It would likely be a few more Ubuntu releases before an x32 version could be called up, at which point it may be largely irrelevant for most desktop/notebook systems.