Just opened mine this evening and sadly found that it was a fake. Will be returning the pad tomorrow.
How can you spot it's fake? If you compare it with your genuine pads it should be easy to tell that it's a copy, even if it appears to function.
Some obvious signs include:
1. Lower quality printing of text (e.g. the "SONY" on the face of the controller, or the "DUAL AXIS" on the front/top)
2. Analogue sticks are stiff, and do not move smoothly
3. Roughness along the seam where the top and bottom halves of the controller shell join (It has the same look as something that's been pried open with a screwdriver)
4. Home button is set too deep and pushes in too far
5. The main buttons (Circle, Square..etc) are duller in colour and the plastic is matte, not shiny.
6. The sticker on the rear of the pad is completely different to a genuine device. Side by side the difference is glaring but if you don't have anything to compare it to, the fake label will be larger than the gap it's meant to sit in, it will be papery; not shiny and strongly adhered. When compared with a real pad, you can see differences in the barcode, the lettering and the layout of the label as well as the model number being wrong in the top right.
7. The pad will not function properly, or even at all (although this would warrant a return even if it was the real thing!)
Hope that helps anyone who has doubts about their controller. There are other signs too that I did not have such as a much blacker looking Home button or having "P3" or some other lettering on the home button instead of the PS3 logo. A quick web search will help if you're still not sure.
For the record, I think I had a very good copy. The packaging was almost perfect (the top edge was slightly wonkey), and to the untrained eye the controller looked like the genuine article. Very easy to spot the fakes when you know how though.