How would you make an on-demand patcher for an MMO?

So all the big MMO's (except WOW), have a patcher that analyzes certain files, makes sure they are up to date, and then runs the game. This is good for a number of reasons. Not only does it keep your files up to date, but it allows the patcher to correct files that have been corrupted, manually edited by the user (cheating), and can prevent a reinstall in a LOT of cases.

In addition, it allows you to track which files people are commonly needing to get new copies of. If every time a player log's in, they need a fresh copy of gamedata.phr, which is an art file, then there's a good chance there's something wrong that is causing a corruption of it.

I think i know how to put together a patcher, i just want to run some ideas by some of you who may have actually already made one. The only thing i'm not sure is by what criteria to have the data analyzed.

I've spent most of my MMO time on EQ and EQ2, so i'l use them as an example. Everytime you run one of those games, it analyzes a number of CRITICAL files, such as the exe's used to run the game. However if your having a problem, you can have it do a "Full File Check" as well, and then it checks everything else. But what is it checking? File size, date, header... all of the above? Then downloading everything that doesn't match the accepted criteria?

I'm interested in any thoughts you may have. Thanks.