I wouldn't say generators are any worse than any other eletrical installation. You just have to be careful with the neutral now that you have two separately derived sources.
The smallest generators I've work with are 200kW, so I can't offer much practical advice about residential installation. We use diesel pretty much exclusively, but it's more for damage control and ease-of-support reasons. Above all, I'd say let common sense prevail! Coordinate with your mechanical subcontractor to make sure your conduit and his/her gas lines aren't interfering. As well as the exhaust, you'll also have to be careful about fuel tank vents- they need to be vented 12' above grade and 5' from any openings.
UFC/NFPA1 offers some more codes that NEC is silent on:
http://www.nfpa.org/aboutthecodes/list_of_codes_and_standards.asp [This message has been edited by SteveFehr (edited 01-22-2007).]
