I have been having problems recently with our websites getting dropped out of the SERPs because the spiders are seeing banner links or text links as the index page instead of our actual sites.
I have had one site dropped from Google and now three from Yahoo each engine attaching to different sources.
In essence, they see www.domainname.com/?engine=advertiser&keyword=link, as the canonical domain... but the real domain is the one with all the inbound links and it gets pushed out as dupliacte content.
I have the two tracking/analytic companies we use see what can be done with this, but a straight 301 redirect strips the tracking code so I lose that...
Anyone else experiencing this???