[BioC] Genbank to Unigene IDs

[Gordon Smyth]

I have a list of GenBank IDs for which I'd like the corresponding Unigene 
cluster IDs. What is the easiest way to do this using Bioconductor 
functions? (I've scanned annotate and AnnBuilder help and vignettes, 
although way too quickly.)

For the sake of being specific, here's a concrete example. What's Unigene 
for GB="NM_004551"?

Thanks a lot
Gordon