there is a possibility the utility may have a transformer too small or the conductors to the property are too small.
there is also the possibility that you have undersized the conductors, possibly due to length (voltage drop).
There is also a possibility both situations have occurred.
Gar, as you pointed out, it has to be determined if the VD is excessive before you proceed with corrective action.091022-2317 EST
I conjecture that in most residential cases that you will see some flicker in an incandescent lamp when a whole house air conditioner turns on.
The real question is -- is the amount of momentary voltage drop excessive?
You need some instrument that can record voltage with a response time of probably 1 millisecond. Even 1 second may be sufficiently good in some cases.
I think most people will detect a momentary voltage change of 3% with an incandescent lamp. My sensitivity is somewhere between 1 and 2%.
If the voltage drop is not excessive for the transformer and wiring impedances, then try GE dimmable CFLs and see if the problem goes away. These have moderately little light change with voltage change down to about 100 V where they drop out.
Keep in mind that dimming lights may imply wiring problems. Especially if some lights momentarily brighten. This is not likely a test in the case of a 240 V air conditioner.