How to specify last column?

[Sarah Henderson <sarah.henderson@ubc.ca>]

Hi all --

I'm sure this is a simple question with a simple answer, but I can't find 
anything in the language guide about it (I'm probably looking in the wrong 
places...)
Is there some trick I can use to assign a vector to the last column of a 
data frame without knowing the column's rank?  Something like:
my.data.frame[END] <- new.vector

Thanks!

Sarah