One of the goals we have for WCF in Orcas is to extend our feature set to better meet the needs of the "plain old XML over HTTP programmer". In other words, we're all about making the experience of building these types of apps better on our platform.
Mashups are in a holding pattern until the next big breakthrough, IMHO, this will likely be design patterns or tools (and probably both) that breaks the logjam preventing easy creation of good mashups.