presidential immunity from suit is discussed here
http://www.nytimes.com/1997/05/28/opinion/the-limits-of-presidential-immunity.html
Immunity was only examined in 1982 - golly that is late !
we did it around 1600
and Clinton ( "I did not have sex with that woman" ) was impeached for it
however it seems s/o else was allowed to indict him for sexual harassment - see NY times article
and it all depends on whether he is following his presidential duties
so for example he could be screwed for shop lifting - - - or tax evasion
and calling Obama not American-born would seem not to be
and also alleging that his phones were tapped whilst he wasnt president - doesnt sound very presidential to me
so perhaps boys and girls - he can be ( sued for libel) or could be
[the First Amendment thingey still stands and acts as an almost fatal bar]
{ damn I couldnt squeeze the phrase 'parens patriae' into this discussion - just to vex the usual suspects]