There are two entries in /sw/etc/fink.conf whose values seem to refer to a Mac OS X version.
The Distribution entry must contain the OS X version of the machine, so if you installed Fink on Snow Leopard, it should be 10.6.
The SelfUpdateTrees entry is different. For Mac OS X versions prior to 10.5 Fink used to organise package descriptions according to the OS X version, so there used to be different trees: 10.2, 10.3, 10.4. Whenever a user ran
Fink would download the package descriptions of the tree corresponding to the specific OS X version where it was installed.
With the release of Mac OS X 10.5, Fink decided not to have different package description trees any longer and reuse the 10.4 tree instead. Therefore SelfUpdateTrees: 10.4 is the correct entry in fink.conf on Mac OS X 10.4, 10.5, and 10.6 because the 10.4 package description tree is shared amongst these OS X versions. This might change in the future.
Note that in general users shouldn’t change the Distribution and SelfUpdateTrees entries. These are created when Fink is installed and updated, if necessary, when Fink is updated.