Alright, you actually had me interested. A plausible demonstration of SSB (single strand breakage) or DSB (double strand breakage) after exposure to the type of RF that cell phones emit would be huge. And to his credit, that's exactly what Lai's results indicated (H Lai, NP Singh - Int J Radiat Biol, 1996).
Specifically, Lai found DNA breakage in the brain tissues of rats immediately after a 2hr exposure to a 2450MHz CW (continuous wave) signal, but not immediately after exposure to a 2450MHz PW (pulsed wave) signal. DNA breakage was only found 4hr after exposure to the PW signal ended. Lai didn't provide any temperature information, so we can't know if that was a factor.
Here's my problem: Lai's experiment was reproduced at least once, and found no significant difference in SSBs between the irradiated group and the controls (http://bit.ly/9qr2Hm).
This is on top of numerous other studies that have failed to confirm DNA damage from RF exposure (http://bit.ly/c9H4MR). I'm not saying that RF-induced DNA damage couldn't possibly happen, but the evidence for it seems weak or non-existent at best.