"I have tried several MICs and resistors, and these are my conclusions:
"The electret MIC always sinks the same current; different values of resistors does not seem to affect this current. Dear audioguru: using my mics, there is no different in this current using a 6.8k resistor or a 10k resistor or a 4.7 resistor.
"I suppose that the quid is to use a resistor that, crossed by the previous current, will generate the optimal voltage jump to place the mic in its standard operation voltage." jfmateos
The experimenting I did with a cheap Radio Shack electret showed pretty much the same results. With either a 1k or 2.2k current limiting resistor in series with the mic, and applying 3VDC and then 10VDC, the current only rose from .172mA to .194mA with either resistor. I guess the mic is happy as long as it can get what it needs...typically .5mA, but less with some models.
On the other hand, the Rod Elliott article that Nigel's last post linked to describes pumping up the sensitivity/output of electrets by applying 9 volts or more to them.
Fascinating little critters, aren't they?