Karen--

More than likely, the mechanic didn't bleed your system of air
properly.  The light is a low-level indicator.  It has nothing to do
with temperature.  It is simple enough to do yourself, but I'd make the
mechanic do it on principal since he didn't do it properly to begin
with.  Also, the next time you get your coolant flushed, he might be a
little more likely to do it correctly if you point this out to him now. 
Also, on the slim chance that his work was not done properly concerning
the heads, if you let him fix the air bubble problem, then he can't
blame you for fiddling around and causing a problem which was really his
fault.

Tonya
87 Vanagon GL Wolfs
74 S. Beetle