Adobe announced that the upcoming Lightroom 6 will require a 64-bit OS in order to work:
"In order to leverage the latest operating system features and technologies, Lightroom 6 will require Mac OS X 10.8 or above, or a 64 bit version of Windows 7, 8 or 8.1. Focusing our work on more modern operating systems and architectures allows us to spend more time adding functionality requested by users, including additional advanced imaging features and improving general application performance."
Here is a recap of what to expect in LR6: