As you know, Lion locks files that have not been changed for 2 weeks, so that you have to unlock them before editing them.
There is a control to change the length of time before a file is ‘locked’ by Lion.
In System Preferences » Time Machine » Options, there is a control to set the length of time after the last edit before files are locked.
The options are 1 day, 1 week, 2 weeks (default), 1 month, 1 year.
I presume that there’s a corresponding plist setting that you can ‘defaults write’ to values not in the dropdown list.
[crarko adds: This is one of those things that's obvious once you know about it, but probably wouldn't think to look for it under Time Machine preferences if you didn't.]