I got exactly those FAA licenses you mention and will start converting them to JAA licenses sometime in mid-2009, simply because FAA tickets don't get you any jobs in Europe if you're low houred, same as JAA licenses.
Most N-reg'd aircraft in Europe which you could fly commercially are bizjets ( = not really good with low hrs ) and a couple of PC-12s and smaller stuff, so I wouldn't count on it.
If you have the right to work in EU countries only, stick to your JAA licenses. A FAA ATP in addition to your JAA license may make more sense when you got a couple of thousand hrs, so you could fly for more operators if you feel like it - contracting for example.
Good luck
INNflt