If you have a residence permit that issues from a Z visa, then you can re-enter the country at any point until the expiration of the visa.  Please check the expiration date of the visa carefully before doing so, however.

yep. 'tis the nature of the RP.  sort of like you legally live here long-term and as such u can avoid the bloody hassles of those staying here on a visa. supposedly upon returning to this fair land you are supposed to go see your local coppers and tell them you have been out of the country, and that you are back now. I've never done this myself, but I think my employers have done it for me once or twice. Perhaps other's can clarify this issue.

the resident permit has somewhere within it a section (just like the visa on your passport) that it is label "No. of entries".  If it says 1, you are allowed to enter once, if it says 2, guess you by now will know you are allowed to enter twice, and if it says M then you can enter as many times as you please (it means multiple entries).  At least in Guangxi Province it works that way for Z visa or L visas (if married to a local), and the cost of the permit goes up with the number of entries.

If in your permit, you do not see this statement, or it is left blank, I strongly suggest you take a quick trip to PSB and ask them how many entries do you have to avoid future problems. 

The residence permit won't saying anything about number of entries.  It will simply list the pertinent information about why you are residing in China, where the permit was issued, expiration date, etc.  You can enter and leave China as many times as you please with the residence permit.  I usually enter/exit China 5 or 6 times a year with no problems.  Just make sure to start the renewal process BEFORE the expiration date.

 A residence permit allows you to come and go from China