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.

Jim S.