[R] extracting index list when using tapply()

[hesicaia]

Hello, 
  The quick version of my question is how can I extract a matrix instead of
a vector using tapply()? I would like to be able to access both the results
of tapply() and also the index variables.

In case further explanation would help:  I am analyzing a large (3million
rows x 9 columns) spatial/temporal dataset and am attempting to calculate
the number of unique years containing any data within each geographic area
(10 degree cells in this case). I can do this, but I also want to extract a
subset vector of the index variable (area).

My script to calculate the number of unique years containing any data for
each area is: 
x<-tapply(years, area, function(x) length(unique(x)))

Now, I want to extract the vector of areas where the number of unique years
containing any data is >20, but tapply() only returns a vector of unique
years and I was a matrix. 
 
I could use a looping function to do this, but tapply() is much faster with
large datasets and so I would like to use it if possible. 

Any help is appreciated.
Thanks.
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