Revision numbers or software versions?
AFAIK, revision numbers are automatically assigned by each commit to the repository.
As for software "versioning," it's largely up to you how to do it. Quite often I see the format as something similar to major.minor.build.revision.
I personally follow a format something like this:
major.year.month.day[.minor/build/whatever I feel like at the time]
Although I haven't been completely consistent with that system. In one case
I've actually done major.build.year.month.day. You can click the little icons in my signature for more examples, if you feel so inclined.
I just like the idea of the date being in the version so you can easily tell at a glance how long it's been since the last update. But it does lack the simplicity of being able to say "Upgrade from 1.2 to 1.3 now!" I'd have to say "Upgrade from 1.2010.10.01 to 1.2010.10.05 now!"
EDIT: Spelling and clarification, etc.