My 7D ignores the shutter speed set in TV mode. So if I set it at 180, or 250, it fires the flash but at a slow speed of whatever the non-flash exposure be, say 1/3 sec in a low lit room. Manual works fine, AV just uses 1/60 all the time.
If the shutter is set to over 1/250 s, it will drop to that value (the max sync speed), but else, it should be manually set between 1/250 s and 30 s. However, the camera will always expose for background, since the metering is done without flash activation. So...if you have Safety Shift (C.Fn I-6) enabled, in Tv mode it will override your shutter speed setting to obtain a 'correct' exposure without considering the flash (and if you need flash, the resulting shutter speed will likely be pretty slow).Also the built in flash fires along with the slave 430 even when it is set to fire just the slave.
The 7D controls the slave flash(es) via light pulses from the pop-up flash - that occurs before the exposure, so the light from the onboard flash will not contribute to the exposure.
Thanks Neuro, I will check that Safety shift setting. I understand about the pop up firing the slave, I thought it was radio frequency though.