Time Machine can help you easily find the place in your backups when a file was changed. Start Time Machine and select a file in the Finder whose previous version you'd like to locate. If you put the Finder in List view, then you can see the preview for the file, including modified date (and version for an application).

Click on the big "back in time" arrow once to search through the list of backups. Time Machine will automatically stop at the point where the file was last changed. If you click the back arrow again, it will stop at the next previous change point. Likewise, the forward arrow scans towards the present for changes.

As an example, I applied the 10.5.1 update today. I started Time Machine and went into my Application folder and selected the Mail program. It showed version 3.1. I clicked on the back arrow, and I was taken immediately to the point where the program version was 3.0, just before the update had been applied.

Another thing I noticed is that if you Command-click one of the arrows, it takes you only one step through the backups, overriding the "scan for changes" behavior.

[robg adds: While I somehow know this is how Time Machine works (though I wasn't aware of the Command key override), I think it's because I saw it in an Apple presentation somewhere. I can't find any reference to this behavior in the Time Machine help file, nor on Apple's list of 300 new Leopard features.]