Jack
Your assuming your inductor is ideal. Iron cores lose inductances
at high frequences as a few hundred hertz in the case of transformers.
I like another picture of what is happening. Most modern dimmers use
scr's or their AC cousin the traic, these are switches that can turn on
suddenly (~microseconds) and turn off when the voltage accross them reachess
zero. So during each ac cycle they turn on suddenly, Its this fast turn on
high frequency edge that is supply current faster then the iron domains
can react that the problem.
I hope this helps.
Gary