I tried evolution-2.24 and it seems it provides acces to the webcal feeds, so probably the evolution-webcal package is not needed any more.
It can probably be ditched, but I'll do so for Fedora 11.
Well, but it may be source of errors, if this add-on does the same as what is already in the main package (actually the evolution-webcal DOESN'T implement reading webcal streams for VJOURNAL/memo component, where the main package does...)
Obsoleted by evolution-data-server-2.25.1-2.fc11.