Hi,
This would affect directory based projects. It would be convenient to be able to change/set which files in the structure are ignored or seen.
This could be based on an individual file. This could be based on (perhaps) a regular expression.
It's been available for a long time, using a regular expression: http://lists.macromates.com/pipermail/textmate/2004-October/000597.html
And you can also change the default regular expression for the projects in the options panel if I remember well...
Cheers, Vincent ISAMBART