[R] run function on subsets of matrix

[fisken]

I was wondering if it is possible to do the following in a smarter way.

I want get the mean value across the columns of a matrix, but I want
to do this on subrows of the matrix, given by some vector(same length
as the the number of rows). Something like

 nObs<- 6
 nDim <- 4
 m  <-   matrix(rnorm(nObs*nDim),ncol=nDim)
 fac<-sample(1:(nObs/2),nObs,rep=T)

 ##loop trough different 'factor' levels
 for (i in unique(fac))
    print(apply(m[fac==i,],2,mean))

Now, the problem is that if a value in 'fac' only occurs once, the
'apply' function will complain.
But I think, this can also be written more cleverly using some R style
construct using factorlevels.

Thanks