gn_wendy - 3:26 pm on Mar 25, 2010 (gmt 0)
A site i work with updates with a new sub-category every once in a while. That usually amounts to 10K-50K pages in one go.
Releasing the pages at the same time has never been a problem, even a few years back when the site was very small and had few links incoming.
Getting those fancy and sought-after deep links, that's the problem... As long as you are publishing good, relevant and above all unique content you should be fine.
Are the 5K old pages also going to stay online? Or do they have a "new" home in the new version? How you choose to do this may well have an affect on your rankings - that is if you are tinkering with the pages and URLs already indexed and ranking.