Apart from the obvious "A pawn is a pawn is a pawn", what is a pawn really worth?
For many years Belconnen Chess Club used to run a "Handicap Allegro". The ratings difference between two opponents determined the material handicap the stronger player had to give up. This could be anything from a pawn up to a queen. The material to rating ratio varied between 100 and 150 points per pawn, without a definite value ever being agreed upon. Even at the lower end (every 100 points equalled a pawn) it was usually one of the higher rated players who won the tournament.
However a study of games played between computer program tends to fix the value at the 100 point mark. The study looked at 400,000 games played between programs and calculated a winning percentage as a function of material advantage. It turned out that the advantage of a pawn resulted in a 64% winning score for the side with the extra material. This is the equivalent of the predicted result of a match between 2 players with a rating difference of 100 points.
Now while I think this relationship is reasonable for a pawn difference, I don't know how valid it is for larger material amounts. I certainly wouldn't feel confident giving Rook odds to a player rated 500 points below me, while I would fancy my chances if the position was reversed.
However in a game between 2 human players other factors come in to play. Indeed IM David Smerdon spent his time between rounds at Street Chess last Saturday giving Emma Guo (last years Australian Girls Champion) Rook odds and beating her easily. The pressure of having the extra material seemed to worry her and she kept dropping material (usually via Bc8-b7xg2 style tricks from Smerdon). It was only when he spotted her 2 rooks that she finally won a game, and even then only by liquidating into a K+P ending (albeit 3 passed pawns up).