Would regular travelers who fly more than five or so times a year be at higher risk for cancer, or other diseases as a result of going through the backscatter xray? (assume every time for the purposes of discussion).
Frequent fliers will be at higher risk for cancer and some other diseases because they will receive above-average doses of ionizing radiation. However, most of this would occur during flight, as the cosmic rays don't have to pass through as much air before they can damage your cells. According to a 1989 paper, the typical dose received would be between 0.2 and 9.1 mSv, depending on duration, altitude, and latitude.
An open FDA letter says that backscatter X-ray (BX) scanners deliver 0.25 µSv or less, so even if one distrusts the government and believes all the sensational articles about scanners doling out 10 or 100x the "normal" dose, you'd still end up receiving that on the flight.
There are those who claim that X-rays are being concentrated in the outer surface of your skin are on dubious ground, as the actual absorbed dose penetrates much further--this letter from a BX manufacturer says it takes 2 inches of tissue analog (plastic; loaded with C-H bonds) to absorb half of the dose. This makes some sense physically (been a while since my physics courses), though I cannot find a good paper about it.
There is a nice article about X-ray backscatter scanners on Ars Technica. As Nick already wrote, the dose delivered by the scanner is smaller than what you receive from flying itself.
A group of professors from the UCSF wrote an open letter about the risks of X-ray backscatter scanners and the FDA replied to it. The main argument is that the X-Rays are more concentrated in the skin as they don't penetrate as far as in conventional X-ray methods. The UCSF professors argue that due to that fact you can't just apply the usual safety limits as the radiation is concentrated differently. They ask for more detailed studies before the scanners are adopted.
They also raise a second point, that the scanners use a fast moving high intensity beam and that an error that would stop the beam movement could lead to a high dose of radiation in a small area.
There is no such thing as a safe dose of ionizing radiation. Consequently, increasing the amount received will increase the danger level some amount. The question is whether it will be worth it.
One quick estimate on Bruce Schneier's security blog was that these scanners would probably kill 5 to 10 people per year (16 per billion scanned passengers). This is extremely small compared to the number of people flying in the US, but it's still higher than the number killed by any attack these would have prevented recently.