Currently Being ModeratedNov 11, 2012 9:09 AM (in response to Flo_Sch)
This issue was first noted here around the beginning of August, 2012. See this thread, but don't take the reply marked "solved" too seriously—it turned out to be premature.
Briefly, the issue is that Numbers consumes about 70% of available CPU time/activity whenever the mouse pointer is on a table. That CPU usage consumes power, creates heat, and activates the fans.
No solution as yet. A summary of suggestions:
File a bug report (Numbers menu > Provide Numbers Feedback)
If you're near an Apple Store, take the MBP to the Genius Bar and demonstrate the issue using your machine (or any 2012 MBP with Numbers installed).
Move the mouse pointer off any table whenever possible.
Check the thread for any I've missed.