[R] Applying functions to partitions

[Titus von der Malsburg]

Hi list!  I have a large matrix which I'd like to partition into blocks
and for each block I'd like to compute the mean.  Following a example
where each letter marks a block of the partition:

     a a a d g g 
     a a a d g g
     a a a d g g
     b b b e h h
     b b b e h h
     c c c f i i

I'm only interested in the resulting matrix of means.  How can this be
done efficiently?

Thanks!  Titus