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For connection through a pc-socket, Canon lists the maximum safe trigger voltage in the manual (e.g. 250 volts). However, this number does not apply to the hot-shoe. Canon does not officially give out information on the safe voltage for hot-shoe mounted flash units (beyond the obvious recommendation that you should only mount one of the Canon's own Speedlites in the hot-shoe).

However, an email from Chuck Westfall (Director, Media & Customer Relationship, Canon USA), posted in this thread in DPreview's Canon EOS 350D/300D forum in April 2005 by Doug Kerr had the following to say about trigger voltages:

The EOS Digital Rebel XT [350D] 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 [300D], 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 [300D]. 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.

In a 2007 interview with The Digital Journalist, Westfall confirms the above and adds some older film SLRs to the list.

I take this to mean that all Canons DSLRs newer than the 350D, as well as all the professional models, can use flash with trigger voltages up to 250 volts in their hot-shoe. However, 6 volts is the safe limit for the D30, D60, 10D, 300D, and Canon's digital compact cameras.