[ savvis folks asking which versions have the "bug"]
we just found that 11.1.7ca1 contains the "bug" but we haven't
gotten an exact description of what the bug is. in general terms,
it's only been described as:
"a bgp router can be slow in withdrawing prefixes even though they've
been withdrawn from the originator(s)."
doesn't say much, does it? 
they're working on a fix. i'm afraid i dont' know which other
versions have the same problem though. we only asked about a
particular version we run.
-brett