I noticed this error a couple of days ago, but I've only really had the time to start looking at the forums for it this evening.
I note that just as I start searching to see if anyone else was having issues, a new version of Jetpack has been released.
So, have upgraded from 1.6 to 1.7 just now, but I'm still seeing this error when I try & get my site stats
invalid_signature: The timestamp is too old.
Can anyone help?