aggregate slow with many rows - alternative?

Hi,

I use the code below to aggregate / cnt my test data. It 
works fine, but the problem is with my real data (33'000 
rows) where the function is really slow (nothing happened in 
half an hour).

Does anybody know of other functions that I could use?

Thanks,
Hans-Peter

--------------
dat <- data.frame( Datum  = c( 32586, 32587, 32587, 32625, 
32656, 32656, 32656, 32672, 32672, 32699 ),
              FischerID = c( 58395, 58395, 58395, 88434, 
89953, 89953, 89953, 64395, 62896, 62870 ),
              Anzahl = c( 2, 2, 1, 1, 2, 1, 7, 1, 1, 2 ) )
f <- function(x) data.frame( Datum = x[1,1], FischerID = 
x[1,2], Anzahl = sum( x[,3] ), Cnt = dim( x )[1] )
t.a <- do.call("rbind", by(dat, dat[,1:2], f))   # slow for 
33'000 rows
t.a <- t.a[order( t.a[,1], t.a[,2] ),]

  # show data
dat
t.a