Here is a good post from the Google Webmaster Blog about open redirects and how redirects as a parameter in your URLs can harm your Google ranking.
As we know Google has to (and it can’t stop) indexing or at least assessing all it sees. It (as well as all the other search robots) trawls the web in search of pages, links, sites, etc. In doing this it follows all the routes it can (or rather, is ethically allowed to).
This means that if it is OK for it to do so (i.e. it can gain access and is allowed) it will seek and find many (many) different ways to find those all-important web pages that keeps it ahead of the competition and up to date. This means that proxy servers and indeed server logs can be open to these information hungry creatures (OK, I hung on for as long as I could before I personalised a search robot, sorry…).
So, taking this into account if you are a victim of someone using your URL as a cloak to get people to click on a link on the web or an email link e.g. www.yoursite.com/go.php?url=www.badsite.com. Then don’t be surprised if Google finds this (and as mentioned it can be just because someone has clicked on the link from an e-mail and it is now stored on a proxy servers, etc. not just because the spammer has set up a web page with ‘click here’) and then indexes this URL. If it does and ‘www.badsite.com’ (great name isn’t it) is… well a ‘bad site’ with porn, malware, SEO black hat, whatever, etc… then unfortunately Google will probably take your site out of its index because of this. This is because you are and your site is (unwittingly) providing a bad URL to Google and this needs to be eradicated. If you are registered at Google Webmaster Central you will get a message telling you what they have done and some instruction on what to do next. If not you will probably be confused as to why your site has been dropped (best bet is to look at your server logs and see which pages on your site have been accessed or use the ‘site:www.yoursite.com’ command on Google with some date parameters added to filter for newly indexed pages).
Anyways, have a read of the article here and see if your site could be at risk.