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The Bureau of reclamation has some outstanding info.  See:
http://www.usbr.gov/power/data/fist/fist3~1/3~1_cont.htm

IEEE 43-2000 is the standard for insulation testing of rotating machines.

According to IEEE 43-2000, 2.0 if the minimum limit for motors with insulation Classes B, F, H.  For class A (only very old motors, the limit would be 1.5).  Also if you have pre-1970's motors with asphaltic insulation systems, I think there may be other limits applicable.

EPRI recommends a maximum 1% winding resistance "imbalance" among the three readings, which they define as
imbalance = (Max-Min)/average.  That value may be a little conservative, so failure to meet that criterion doesn't necessarily mean the motor has a problem, only that you need to investigate some more.

Good answer electricpete. I would like to add that one must be careful with winding resistance as it is very temperature dependent. A motor with a heater(s) in the stator will almost certainly give erroneous readings as the location of the heaters will probably affect the resistance of one or two phases as opposed to equally affecting all three.

Also, a more liberal (maximum) acceptable value for winding variations, whether inductance or resistance, is +/10% of the average value.

Suggestion: Visit
http://www.glacierbay.com/Insultest.htm
http://www.dirnrg.com/Products/dielectr.htm
http://www.phenixtech.com/
http://www.ieee.org/web/developers/webthes/00000228.htm
etc. for more info