Just some links for me, sorry. Expect my personal definition for Mobile 2.0 a bit later.

Mobile 2.0 is not Web 2.0
Experts: Web 2.0 doesn't copy directly to phones

"Beware of naive copying of PC services," said David Wood, executive vice president of research for Symbian. "Some don't translate." [...] Wood and others said that the inherent constraints of mobile phones and networks mean that many Web 2.0 services won't work well without some changes to accommodate those limitations.

[...] Wood expects to see growing interest from Web 2.0 companies in the mobile space, in part due to "intense competition" for users on the Web. But he warned application developers not to regard the smartphone as "an impoverished version of the PC."

Mobile 2.0
Mobile Web 2.0 by Ajit

See also:
Mobileajax