It is kinda weird that the post went unanswered after a couple of years!
http://forums.collectors.com/message...eyword2=pegleg
Here's a little post although it does not concentrate specifically on the 1971-S Pegleg Proof it does give some definition on what a pegleg is.
To repeat, the R in LIBERTY on your typical Eisenhower Dollar has serifs on the left leg of that R. After a certain amount of die abrading and repair, those serifs invariably get polished off leaving that left leg straight up and down. Extreme cases have the leg of the R actually fading into the field.
The proof IKE is a different story though as these coins are peglegs by design, similar to the 1972-S Proof and 40% Business Strike coins which are all peglegs. Some of them do fade into the background or appear extreme due to a lack of cameo frost though.

A 1971-S Fading Pegleg Proof

A 1972-S Proof
The 1976 BiCentennial coins (Type 1 and produced in 1975) are peglegs as well. When these get abraded, the R really fades!
Peglegs do not have any appreciable value over their non-pegged counterparts with the exception of the 1971-S 40% Business Strike coin. This sucker is more rare than a Type 2 IKE although current prices do not reflect this!
Peglegs do come is all different kinds of varieties though:

A 1974-D

A 1972-D

A 1978-D

A 1972 Type 1
This 1972 Type 1 is more by design that actual die abrading.
Most peglegs of the die abraded variety originated from the Denver Mint primarily because they had to make their dies last and last. Unlike Philadelphia (who distributed dies to Denver) they could just make another one up so die repair was not real high on their list. The end result is some very nice business strike peglegs from Denver!