Usually the author (that posted it on Dribbble) will have a description of how he likes the PSD to be used - ie: Not for redistribution or doesn't want to see it used and reposted on Dribbble etc.
Safest bet - contact the author directly and clarify. You can find their contact info through Dribbble.
Personally when I find resources I'd like to use (mainly non print, commercially), I contact the authors directly and offer to buy a license. Everyone's pretty open and friendly.
Half the value of these sorts of resources is in having a clearly displayed license. If you're managing such a service why would you not require people to specify a license at upload (which could include "must contact me at user@example.com") rather than have each individual have to make the effort of establishing the license conditions?
I used http://www.stockfreeimages.com/terms.html recently, they have a clear statement of terms and each image download page has the copyright details on it; handy.
