Bicycle Mechanics - Is my rear derailleur 10 speed compatible?

I have a 2007 Giant OCR 2. The rear dérailleur is a Shimano 105. I believe I saw somewhere that it might be both 9 and 10 speed compatible. Right now it is a nine speed set up using tiagra shifters.

If I replace the shifters with ten speed shifters will this dérailleur work?

Derailler is dumb, shifters control the speeds. No problem.

If I replace the shifters with ten speed shifters will this dérailleur work?

Yes, the derailleur will work with any Shimano index shifters except pre-nine-speed Dura Ace, including 6,7,8,9, or 10 speed. The key is to match the ten speed shifters to a ten speed cassette, and get a ten speed chain. If you were to stick with the nine speed cassette, there's an alternate cable routing techique on Sheldon's site you might try, which might give you satisfactory results shifting with ten speed shifters and a nine speed cassette. But if you're going to get ten speed shifters, my thought would be to go to a ten speed cassette (and chain), also-

OK, Thanks for the quick responses.

After reading the Sheldon brown article about front dérailleurs, it looks like the only thing I would need to change to go from 9 to 10 speeds are new shifters, a ten speed cassette, and a new ten speed chain.

Is this correct, assuming I would get new cables when I buy the shifters

Don't forget the 10 speed bar tape!