I am a little concerned about doing that though. Having seen sites killed stone dead by people blocking stuff by mistake in robots.txt and the engines then never looking at them again I'm very wary indeed of blocking stuff I intend later to unblock.
In a nutshell, if you put up millions of pages and tell google about it it will index you, if you don't want that you'll have to make choices about the quantity and/or switch to a different kind of host.
Also, this kind of 'bot trap' tends to attract penalties so if this is not some ploy to get traffic out of google you may want to re-consider how you've laid things out, the difference between a legitimate site with a lot of generated pages and a page-spammer is hard to determine and google tends to err on the side of caution.