Thanks all for the replies.
The SF Web-to-Case form through Marketo is appealing to us because of the volume of support requests we get. We see it as another way to track and identify our visitors. I am just a neophyte at this but it seem that if there is something in Marketo for the web-to-lead function, it couldn't be much of a stretch to make something available for web-to-case. But I have been mistaken before 🙂
As far as form validation, we are pretty comfortable setting up some JS code for this. However, I am told that a lot of people turn off JS and, if so, that is a bit of a concern. So, we might end up doing this through our CMS.