The BARB (Broadcasters' Audience Research Board) have done a review of the impact of delayed viewing using iPlayer and similar devices.

With access to SKY+ figures the report includes long-term delayed playback, and retention of programmes.

Whilst 'skewed' towards SKY (as the figures are based around the SKY+ reports..). it makes for interesting reading regarding the most popular programme, and the most popular TYPE of programme.

"Mediatel" have a summary of the report here, by Joe Lewis who is deputy research director at BARB.

BARB's web-site is here.