If you test that the TLD exists... it will still work.
Only if A) you are always online with B) reliable access to the tld's nameserver/s, and C) can deal with the latency. In practice this is often not the case.
Even under the most wildly optimistic scenarios, it's hard to imagine new TLDs being added more than once a month, and I presume that everyone here already knows how easy it is to get copies of the root zone. The reasonable way to validate TLDs is to fetch a copy of the zone every couple of weeks and cache it. By the way, to be sure we're all on roughly the same page, here's a quiz. How many names are there in the root zone right now? a) 11 b) 97 c) 153 d) 280 e) 974 Regards, John Levine, johnl@iecc.com, Primary Perpetrator of "The Internet for Dummies", Information Superhighwayman wanna-be, http://www.johnlevine.com, ex-Mayor "More Wiener schnitzel, please", said Tom, revealingly.