See what process is using a file in Mac OS X

That's simple: sudo fs_usage | grep [path_to_file]

lsof will list open files, but it can be a bit awkward for momentary touches (eg, if the file isn't open when lsof runs, it doesn't show).

I think your best bet would be fernLightning's fseventer.app. It's "nagware", and allows you to watch (graphically) the fsevents API in real-time.

But I spent 2 minutes Googling and found your answer here.

$ lsof | grep [whatever]

Where [whatever] is replaced with the filename you're looking for. With this, you can see which program is desperately holding onto your about-to-be-trashed file. Once you exit that program, your trash will empty.