"Canon US has verified (to me, and here) that the Powershot G doesn't like voltages over 6V.
"Similarly, Nikon has specified 12V for their speedlight circuits... (though reader Steve Francesoni called Nikon.uk to check, and their tech rep said that his N80 was good to 250V — so there may be more complexity to this story). I've heard some rumours that some Coolpix models have been restricted to 5V! (see below for more details)
"Marco Fortin-Metzgen checked with Olympus Europe on his C4040 — that digicam has a trigger voltage of 10V, so Olympus too recommends strobe triggering in the 3V to 6V range.
"Pentax users may want to read this related story from Gene Poon.
"Ron Alexander claims his Fuji is astonishingly tolerant of high voltages... interesting (This has since been verified by Rob Scrimgeour of the FujiGroup.net forum — their members got a message from Fuji also stating the 400V center pin limit).
"(According to some opinions, high voltages can even endanger mechanical cameras, albeit after years of use)
"The ISO 10330 specification ("Photography -- Synchronizers, ignition circuits and connectors for cameras and photoflash units -- Electrical characteristics and test methods," 1992) says that all ISO-compliant cameras should be able to accept trigger voltages up to 24V. Though a Canon engineer is the nominal head of the ISO workgroup, for some reason Canon continues to insist that their cameras tolerate no more than 6V (make that Canon USA — an email from Canon Canada says: "There is not a maximum voltage requirement for the hot shoe terminal on the PowerShot G1." Go fig!). For that reason I've tagged strobes that trigger at voltages between 6V and 24V as "your call." Depending upon who you ask — the camera or strobe manufacturers — those strobes are acceptable or they are not.
"The ISO spec doesn't really seem to hold a lot of weight!"