Good to know... I used to turn time machine off every time I cracked Lightroom open... I thought I remembered this being a thing I didn't have to worry about anymore starting w/ v3? I just dug up the thread that convinced me:
The safest option sounds like just excluding my catalog file manually in the TM pref pane, but perhaps it's a bug that LR4 isn't excluding the catalog itself while running, like LR3 did? I never excluded LR3 from my TM backups...which may have been unwise...but never ran into problems like this.