Most new changes in 0.5.3 are robustness and portability fixes and a working Windows port, the vast majority of it being done by Matthias Gehre.
Coverage of the testsuite from the Tamarin VM is also progressing well.
The source tarball and .exe installer are found on Launchpad
Ubuntu user can get updated packages from the daily PPA for Natty and Oneiric.
You can follow the development version and contribute on github.