I spent a lot of time today trying to figure out the 50,000th download of a fulltext on one of our customer's repository.  I have created a script to do this (whether you have IRStats or not) and I've blogged about it, as I haven't actually figured out how to tell the 50,000th download with any degree of confidence.

See:

If anyone can do any better than me on this, please let me know.  I may have missed something, but it was interesting to explore this.