See what process is using a file in Mac OS X
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.