[BioC] Minimum expression threshold?

[Bob]

Hello,
I have started using bioconductor (which is great, by
the way), and I have a question regarding how to
choose a minimum expression threshold.  

I have read in the Affymetrix cel files, calculated
expression using rma(), and now have a data frame with
~22k expression values across 14 samples (using the
U133A chip).  There are expression values for each
Affy spot - although it is probably not true that this
tissue expresses all 22k genes.  My question is how do
I choose a threshold above which I consider the gene
to be expressed?

In addition (please correct me if I'm wrong), using
only the number of expressed genes (or at least not
all of the spots) will make for better values using
the multtest package.

Can someone point me in the right direction, or point
out some good references on this topic?

Thanks.