I've increased the minimum version of SWIG required to build IMP to 1.3.34 (it was 1.3.31 before). This is needed because the infrastructure to handle reference counting properly was only added to SWIG in 1.3.34.
All of the lab machines run SWIG 1.3.36, so you shouldn't notice any difference. If, however, you're building IMP on a non-lab machine, you may need to upgrade your version of SWIG.
For example, Macs running 10.5 ship with SWIG 1.3.31, so you'll need to install a newer version, either with Fink or MacPorts, or by compiling it yourself. If you install the MacPorts version, you'll need to add path='/opt/local/bin' to your config.py in order for scons to pick up your newer SWIG rather than Apple's copy in /usr/bin/. To compile from source just download swig-1.3.36.tar.gz, extract it, then run "./configure --prefix=/usr/local && make && sudo make install" or similar. (The advantage of building from source yourself is that the MacPorts package takes forever to build because it seems to have a lot of not-really-necessary dependencies; the disadvantage of course is that the resulting installation is not controlled by a package system.)
The actual fix to get the IMP Python wrappers to use this functionality to do reference counting properly will follow once we have ironed out any problems that crop up with the newer SWIG (if any) and I have had a chance to adequately test the new code.
Ben