Which type of url should I use for a mobile website?

Question 1

Should we give the access to mobile version to all users on desktop. by having a differetn url like http://m.yahoo.com/or it's good to keep mobile version and desktop version access only from mobile and desktop like http://sl.dk. Although advanced user can see by changing UserAgent?

Question 2

If we decide to give different url to mobile version and will redirect from main domain to mobile version if user is accessing from mobile then how i should keep url ? though desktop user can also see the mobile version by typing mobile url.

1) Keeping mobile version in sub folder like http://bagcheck.com/mobile

2) With m. subdomain like yahoo http://m.yahoo.com/

3) or like http://mobile.twitter.com/

4) or a totally different url http://website.mobi