I know this has probably been asked and answered before, but I wasn't able to find the thread.  Is there anyway to have MT default every entry to a specific category so that if the author posts without selecting a category it will be placed in the default.

For example, you could have an "uncategorized" category (what an oxymoron) and any entries that were posted without having their primary category changed would have the post default to "uncategorized"

I have no knowledge of how you could set this, I suppose it could be done with a hack into the code but I don't delve that deep.

Go check the suggested features board and see if that is on there, it's a good idea and should be pursued.