<p>I've been playing around with the ASP.NET MVC Framework and the one thing that's really confusing me is how I'm meant to do server side validation of posted form data.  I presume I don't post back to the same URL, but if I don't, how do I redisplay the form with the entered data and error messages?  Also, where should the validation logic go?  In the model or the controller?  This seems to be one of the few areas where web forms are much stronger (I miss the validation controls).</p>

Have you taken a look at this?
http://www.codeplex.com/MvcValidatorToolkit
Quoted from the page
The Validator Toolkit provides a set
  of validators for the new ASP.NET MVC
  framework to validate HTML forms on
  the client and server-side using
  validation sets.
I'm afraid that someone more MVC-savvy than me would have to speak to where in the architecture you should put things.