Before wasting too much time messing with cables and blaming USB ports (like I did), check on Apple’s Software Update if the iPhone SDK Compatibility Update 1.0 is there for you to install.

The solution to your problem is just a handful of kilobytes and one restart away :)

Here is the relevant post on Apple’s support website:
Mac OS X v10.6: iPhone or iPod touch not recognized if iPhone SDK was installed in Mac OS X v10.5