It's fun to discuss idealized TLDs.
Overall IMHO we only need one system of TLDs for the internet. The Country Code System is the most logical and should have been the only allowed first level domain. Then each country can create custom second levels to suit it's needs such as "gov", "edu", "com", "mobile", "ect". The end user could have their domain at the third level or second if allowed. I think things were happening somewhat in this fasion at one point. For the United States our mobile domain should be "whatever.m.us". Please do be too harsh on myself for this opinion but I think it's the logical addressing system. Mathmatic like logic should be used at the first level and branding kept to the second or third level. So under my ideal system .com is king no more but would .com.us be king? LOL
Back to the .m vs .mobi, yes dot mobi is far more brandable but hell to type. "m" and "o" are on the same key which will often make a typo. A default mobi key will be our only hope IMHO because most devices will not likely be qwerty any time soon. I am a mobi owner and developer but admit it sucks to type .mobi on any non qwerty keypad and .com suck almost as bad too! I just want to type ".m" and go to a mobile site without all the branding.