In a recent Google Groups thread many have questions about the remove URL feature from Google. Just a couple of days back Matt Cutts, released a video explaining the removal of URLs, see:Matt Cutts, Video: How to Remove URLs from Google?.

The facility to remove URLs was talked about by Google last year, in a post by Vanessa Fox on Webmaster Central Blog. The Webmaster Help Center, clearly points out a 90 day period, "Content removed with this tool will be excluded from the Google index for a minimum of 90 days, regardless of whether the content becomes available to our crawler during that time." During the conversation in the thread at Google Groups, "How long does a Removal Request usually take?”, Googler JohnMu explains the 90 day period and the reason for that as, “If a URL is marked as "removed" then it should not be shown in web-searches on Google for the next 90 days. In general, if a URL is no longer on your website, it will probably not be reincluded after those 90 days — if it is, you can just request a removal again. The 90-day limit is to help make sure that you do not accidentally remove content from the search results permanently.”

So if you accidentally removed your URL you might want to resubmit it following the steps over here. In case you didn't do it accidentally then you might want to make use of noindex and a robots.txt file to prevent the Googlebot from crawling the page.