It's come up on This Week in Law. Someone was sending back a modified EULA in the POST data.

The lawyers on the show mainly thought it was cute and not something that would be recognized in court.

Sure. My thought is that, if it won't be recognized in court, probably neither will the "I accept" checkbox.

I've implemented electronic signatures before and what I like to do is have the browser submit the agreement back to the server and the server rejects it if it's been altered.

That seems reasonable.