Yes. Presuming you are using custom firmware, set an iPlayer programme playing with the HiDef option - if you have a slow broadband it is likely to be very jerky, you can pause it if you like (or at least turn the sound off, or even change the TV to internal tuner). Once the WebIF media browser reports that the buffer has stopped growing, you can save it off.
However, that said, I tried doing this recently over a 2.5Mbps connection and it took ages, and had a few false starts (the result was an incomplete recording, start again - and you can't use skip forward while you are trying to capture the stream either - that leads to an unplayable file).