Update #2: Today Nikon UK announced a cashback for the D90 and a price drop for the D5000 (source).
Update #1: There are different Nikon promotions ongoing in other countries too, like Sweden for example where the D700 body is currently selling at a 26% discount.
Nikon UK announced a promotion for a free Nikon MB-10 battery grip when you purchase a D700. The offer expires on June 30th, 2010.
Nikon Italy has a promotion for a free Nikon SB-900 when you buy a D700. The offer expires on June 5th, 2010.
The Nikon rebates in the US will expire on May 1st, but I will not be surprised if they get extended again.
Nikon D700 was announced on July 1st, 2008.
Nikon just announced a new lens (200-400mm).
Photokina will take place at the end of September this year.
Based on all of this information, I think it is be safe to assume that the Nikon D700 replacement will be announced in the July-September, 2010 timeframe with July being the best choice.