1. Create a new assembly
2. Add any part to the assembly
3. Float the part so it is completely unconstrained
4. Click and drag the part to move it
5. Click a face on the part
For my SW2009 SP 5.1 and my SW2011 SP 3 installs, the part and assembly are now flagged as needing a rebuild even though nothing has changed. If you click on an edge or vertex in Step 5, the rebuild flag does not appear.
I noticed this in a much larger assembly with mating problems I was trying to correct.