As I understand it, one may use a 20a circuit in a bathroom for all needs, provided it serves that space only; correct ? Does this include heat if it falls within load range ?

Oddly true. It is a horrible design but you could put one of those 1000w fan/heat/light combos on the bathroom circuit with the basin receptacle if you only serve one bathroom.

Yes, The ckt must remain in that bathroom.For lights ex fan etc. thats ok.
As far as the heat goes.. NO.
See: 210.23 (A) Exception.

edit: Greg your fast.
fan/lite/heat combo would be a fixture.

I was thinking wall heat

210.23 (A) allows the only receptacle outlets to be the ones specified in 210.11 (C3).
Unless this is a portable unit, the heat would have to be wired direct and is limited to 50% of the circuit.

See: 210.23 (A) Exception.

I read that to disallow. 210.23(A) 1 & 2 for these areas.

Yes to the first part of you question. Without specific info, I would say no to the second part.
See 2008 NEC 210.11[C][3] exception, which refers to 210.23[A] exception, and 210.23[A][2]. Even if you could, you would be limited to 50% of the branch circuit rating. According to 424.3 though, fixed electric space heating would be a continuous load.
I think most manufactures even recommend an individual branch circuit for the infrared or combo heat/light/vent units as well.
I don’t think it would leave much capacity for those Mega hair dryers and curling irons that all the ladies seem to use these days.

(Kjay) "See 2008 NEC"

Wow! I dodged a bullet there.
Good thing no change from '05.

I'm gonna get the new '08 on Tuesday! Before I get in a jam.:)