For each assembly to be tested, you have to explicitly name it in the NUnit task configuration or the .nunit file.
We would like to use a wildcard pattern (e.g. *.Test.dll) to select files for unit testing, rather than having to edit our task or .nunit file every time we add tests.

For reference, see the TeamCity implementation here: http://confluence.jetbrains.com/display/TCD8/NUnit