Redirection sounds cool, but I am not sure this is such a great idea. I frequently view the full sized version of the website with my Droid on purpose, because the mobile version lacks the full sites info as most mobile sites are just a sub-set of the full sites available info.
By forcing the user to the mobile version, they would get trapped into not being able to access the full site if they want to. The mobile browser my Droid has is quite capable of viewing many full sites, not necessarily as easy (need to zoom in etc), but still able to view it. Even Flash and JavaScripted parts.
Instead, I would use the detection to provide a link that stands out that informs the user of the mobile sites availability and an easy way to get to it.
Its kind of the same thing you can do to notify a user that they have JavaScript turned off. They can use the site, but at least they know why some parts of it might not be working properly.