Write a function to decrypt Feynman Challenge Cipher #1

Golfscript, 10 chars

n--1%5/zip

Takes input from stdin. It may be separated by newlines, as in the question presentation, or not, as many answers assume. (Stripping the newlines is necessary anyway given that it's likely to arrive with one on the end, which messes up the blocks of 5). If we assume that the input arrives with precisely one newline, being the last character, we can shave one character to

)!(%5/zip

Mathematica, 51

Mathematica is quite verbose here, but it is also easy to guess what it's doing.

Thread[Characters@#~Partition~5]~Reverse~{1,2}<>""&

J, 13 characters

|.@(/:5|i.@#)