Today somebody was running the IIS SEO Toolkit and using the Site Analysis feature flagged a lot of violations about "The page contains multiple canonical formats.". The reason apparently is that he uses Query String parameters to pass contextual information or other information between pages. This of course yield the question: Does that mean in general query strings are bad news SEO wise?
Well, the answer is not necessarily.
I will start by clarifying that this violation in Site Analysis means that our algorithm detected that those two URL's look like the same content, note that we make no assumptions based on the URL (including Query String parameters). This kind of situation is bad for a couple of reasons:
Query String by themselves do not pose a terrible threat to SEO, most modern Search Engines deal OK with Query Strings, however its the organic linking and the potential abuse of Query Strings that could give you headaches.
Remember, Search Engines should make no assumptions based on the fact it is a single "page" that serves tons of content through a single Absulte Path and the use of Query Strings. This is typical in many cases such as when using index.php, where pretty much every page on the site is served by the same resource and just using variations of Query Strings or path information.
Well, there are several things you could do, but probably one of the easiest is to just tell Search Engines (more specifically crawlers or bots) to not index pages that have the different Query String variations that really are meant only for the application to pass state and not to specify different content. This can be done using the Robots Exclusion Protocol and use the wildcard matching to specify to not follow any URL's that contain a '?'. Note that you should make sure you are not blocking URL's that actually are supposed to be indexed. For this you can use the Site Analysis feature to run it again and it will flag an informational message for each URL that is not visited due to the robots exclusion file.
In summary, try to keep canonical formats yourself, don't leave any guesses to Search Engines cause some of them might get it wrong. There are new ways of specifying the canonical form in your markup but it is "very recent" (as in 2009) and some Search Engines do not support it (I believe the top three do, though) using the new rel="canonical":
In the Beta 2 version of IIS SEO Toolkit we will support this tag and have better detection of this canonical issues. So stay tuned.
Other ways to solve this is to use URL Rewrite so that you can easily redirect or rewrite your URL's to get rid of the Query Strings and use more SEO friendly URL's.