Transferring Skitch history to new Mac
Looks like Skitch employee, Keith, answered this question on GetSatisfaction.

The short answer is that you need to copy the following preferences files:

~/Library/Preferences/com.skitch.skitch.history
~/Library/Preferences/com.skitch.skitch.plist


This is in addition to copying the actual .skitch files located in:

~/Pictures/Skitch