Google's crawling habits have changed alot in the past 6 months. 3 or 4 months ago
Matt Cutts posted in his blog about supplemental results, spammy sites, and Googlebot crawl rates. I tried to find that post, but his blog has a ton of pages now and I could not locate that post.
In essence, what he talked about was that if there was some reason that Google did not like something about your web site, the Googlebot would not crawl your site as much or as deep as in the past. So if you tick off Google, it could be a while before they grab all of your pages again.
One way to possibly mitigate this circumstance and speed up the crawling is to sign up for Google
sitemaps.
I know from personal experience that it can be very frustrating for Google to re-index all of your pages when you make changes. In my case, it took almost 2 months. But now that the site is back in the good graces of G, the bot hits on a frequent basis.