There was a bug in the initial release of 5.5 (but not in 5.0) which
impaired the intended backward compatibility. Key signatures issued
by an RSA key on another RSA key were using v4 packets, instead of the
intended v3 packets. Older versions of PGP will complain when they
see v4 signature packets. This can cause some interoperability
problems even when using RSA keys.
This bug has been fixed in update release 5.5.2. (Sorry, I am not in the
distribution chain, and I am not familiar with the procedures involved
in acquiring updates.)
Hal Finney