EDIT: I’m a bit late with upgrading to Snow Leopard (just a few days!) but MacPorts has a package available which should work with Mac OS X 10.6.
If you want to use MacPorts on Mac OS X 10.6 (Snow Leopard) and can’t wait for the next release, you best get the latest from the SVN repository (posted on MacRumors):
Get the latest XCode first: the one you installed on Leopard might not work. ./configure will fail not finding a decent compiler.
$ svn co http://svn.macports.org/repository/macports/trunk/base/ macports-base
$ cd macports-base
$ ./configure
$ make
$ sudo make install
$ sudo /opt/local/bin/port -v selfupdate