Low voltage dimmers, as I understand them, are still 120v dimmers that are designed to control the hi-voltage side of transformers that power low voltage lighting. Although they called "low-voltage dimmers" In almost every respect they are the same as standard 120v dimmers. Dimmers that are controlled by a lighting system that have a low voltage control cable running to them, as I understand it, must have a barrier between the low voltage and high voltage wiring separating it from each other. OR, the insulation of the control cabel must be rated 600v which effectivly provides the barrier. If I am wrong on this I would love to hear, as most electrical inspectors I talk to are not that well versed in low voltage and have not been able to give clear direction.
When talking about 'low voltage' dimmers from low voltage systems (not like Lutron 120v to 12v dimmers), you cannot install them with 'other' systems regardless of the conductor rating. Refer to 725.55 of the 2002 NEC for the wiring methods. I spoke to a manufacturer about the 'combo' boxes that are available for 'line/low' voltage, and his advice was to try to keep a 6 inch separation from low to line voltage so as not to corrupt the low voltage signal.
There are two good books available that I know of, Mike Holt's new 'Understanding Low Voltage and Power-Limited Systems and Noel Williams's Limited Energy Systems.