i asked this question to Canon support last week for my Canon 5dM2 I just purchased. Their reply was :
Thank you for your inquiry about the hotshoe on the EOS 5D Mark II.
The hotshoe will safely work with trigger voltages up to 6 volts.
I hope this information is helpful to you. Please let us know if we can be of any further assistance with the EOS 5D Mark II. Thank you for choosing Canon.
Technical Support Representative