The reason why I wanted to know was because I discovered some odd urls in Webmaster Tools that looked like search urls. So to find the answer to this question I first set up a page with 3 forms.
Form 1: Using the form GET method my page would get a new title and content if form was submitted. The URL would be different since I added a URL variable to the original URL.
Form 2: Using the form POST method. If this form was submitted the original page would have new content, just like search page.
Form 3: Using the form POST method but with a different action URL than the form page itself. I did this to check if Google would discover the “secret” action page.
The result was quite disappointing. None of my forms were crawled during a period of two months. I am pretty sure this is standard behavior from Google. However if you want to be absolutely sure your forms are not being crawled and indexed you could always add some robots.txt rules and a meta noindex to the action url.