A more complex version of XmlRpc. (OK that's a bit too terse, it's a more generalized model, providing transport-protocol-independence, among other things...)
I thought (2002-04-11-f) that Zope didn't have much SOAP support, but there are some articles talking about the ZSI.
A Sam Ruby Jul'02 piece on combining ReST and SOAP.
SOAP should allow transport over SMTP (ASynch Messaging), Jabber, etc. - but I believe you should not assume that a given toolkit will actually do so.
I'm not even sure how good the true cross-vendor InterOp is for SOAP.
A July'02 article compared the performance of XmlRpc, SOAP, and CORBA (using Python).
Mark Nottingham on SOAP as format instead of protocol.