Again, I don't encourage anyone to mess around with this without proper confirmation of what the individual unit puts out - and if it may be defective or not. However, I still believe that this has turned into an urban myth to some degree at least. The 5DII is rated at 250V. There may be some very old or uncommon units that exceed that.
The 250 volt figure appears in a online blog attributed to Chuck Westfall. Note that older DSLR bodies have a low voltage rating, but a shutter redesign after the D30, D60, D10 and Digital Rebel, starting with the 20D raised the voltage. So any newer body should be safe.
Chuck Westfall, Director/Media & Customer Relationship, Canon USA, advises as follows regarding the flash sync circuit of the EOS Digital Rebel XT (EOS 350D):
The EOS Digital Rebel XT uses a modified version of the EOS 20D's shutter unit. Consequently, acceptable trigger circuit voltage for both cameras is the same, i.e., 250 volts. Except for the original Digital Rebel, all current EOS digital SLRs (i.e., EOS-1Ds Mark II, EOS-1D Mark II, EOS 20D and EOS Digital Rebel XT) generate their X-sync signals electronically rather than mechanically. This is why they have higher acceptable trigger circuit voltage ratings than earlier models like the D30, D60, 10D and original Digital Rebel. These older models cannot be modified to achieve a higher trigger circuit voltage rating, since such a modification would require a different shutter mechanism as well as a complete redesign of the supporting circuitry.