nomis5 - 7:01 am on Aug 31, 2012 (gmt 0)
I also use screen size, user agent is far too complicated and needs constant maintenance.
On both mobile and full site there are very clear options to let the user go to the mobile / full version of the page if they wish.
The only complication I came across is that if I redirected a mobile user from the full site to the mobile and they then went back to the full site, the redirect code on the full site automatically redirected them back to the mobile site again.
Got round that by dropping a cookie which indicates the user wants to stay on the full site whatever their screen size.