subsetting data frame using by() or tapply() or other

Ok so I see the problem that I'm having creating a new variable (LAG1DBC) 
in the example data transformation below is that tapply() is creating a 
list that is not dimensionally consistent with the data frame (data).  So 
how do I go from the list output of tapply() to create a dimensionally 
consistent vector that can create the new variable in my original data 
frame?  I've been trying to use a function like
data$LAG1DBC <- tapply(data$DBC, data$LOCID, function(x) c(NA, 
x[-length(x)]))
which creates a list of dimension much smaller than the nrows in data. And 
I've tried things like using as.data.frame.array() or as.data.frame.list() 
in front of tapply() and still have the same problem.  I know this can't 
be that unusual of a data manipulation and that someone has to have done 
similar things before.

I want to go from something like this:

       LOCID  POPULATION  YEAR        DBC
1      algb-1           A 1992 0.70451575
2      algb-1           A 1993 0.59506851
3      algb-1           A 1997 0.84837544
4      algb-1           A 1998 0.50283182
5      algb-1           A 2000 0.91242707
6      algb-2           A 1992 0.09747155
7      algb-2           A 1993 0.84772253
8      algb-2           A 1997 0.43974081
9      algb-2           A 1998 0.83108544
10     algb-2           A 2000 0.22291192
11     algb-3           A 1992 0.44234175
12     algb-3           A 1993 0.54089534
5680 taylr-73           B 2001 0.43918082
5681 taylr-73           B 2002 0.34694427
5682 taylr-73           B 2003 3.35619190
5683 taylr-73           B 2004 0.71575815
5684 taylr-73           B 2005 0.42038506
5685 taylr-74           B 1992 3.88410354
5686 taylr-74           B 1993 3.32472557
5687 taylr-74           B 1994 3.29861501
5688 taylr-74           B 1996 0.48153827
5689 taylr-74           B 1997 3.63570636
5690 taylr-74           B 1998 1.94630194

to something like this:

       LOCID  POPULATION  YEAR        DBC LAG1DBC
1      algb-1           A 1992 0.70451575       NA 
2      algb-1           A 1993 0.59506851 0.70451575
3      algb-1           A 1997 0.84837544       0.59506851
4      algb-1           A 1998 0.50283182 0.84837544
5      algb-1           A 2000 0.91242707       0.50283182
6      algb-2           A 1992 0.09747155       NA
7      algb-2           A 1993 0.84772253 0.09747155
8      algb-2           A 1997 0.43974081       0.84772253
9      algb-2           A 1998 0.83108544       0.43974081
10     algb-2           A 2000 0.22291192       0.83108544
11     algb-3           A 1992 0.44234175       NA
12     algb-3           A 1993 0.54089534       0.44234175
5680 taylr-73           B 2001 0.43918082       NA
5681 taylr-73           B 2002 0.34694427       0.43918082
5682 taylr-73           B 2003 3.35619190       0.34694427
5683 taylr-73           B 2004 0.71575815       3.35619190
5684 taylr-73           B 2005 0.42038506       0.71575815
5685 taylr-74           B 1992 3.88410354       NA
5686 taylr-74           B 1993 3.32472557       3.88410354
5687 taylr-74           B 1994 3.29861501       3.32472557
5688 taylr-74           B 1996 0.48153827       3.29861501
5689 taylr-74           B 1997 3.63570636       0.48153827
5690 taylr-74           B 1998 1.94630194       3.63570636

Brian

data$LAG1DBC <- unlist(tapply(data$DBC, data$LOCID, 

data