On a related note, most people I work with use tabs for indentation, so I generally have Textmate set up to do so (with a tab size of about two characters), but then when editing YAML files, I have to manually adjust this. And remember to set it back afterwards.
Some way of YAML files automatically banning tabs and just using spaces would be really nice.
And having the option of doing the same for Python source would rock, too :-)
AFAIK, Textmate should remember your tab settings on a per-language basis...