Timezones of some kind are very important, however referencing another RDF file means that someone has to write the file, and that isn't what RSS creators want to spend their time doing. An alternative is always to use UTC offsets, such as <rdf:value="20020104T17:00:00Z-0500"/> which is consistent with W3C's preferred date and time formats, but not with iCalendar usage.
well maybe not that bad. But not very pretty
It is sometimes useful to separate out date from time and the component parts of time from each other. Separate times and dates arer available in the core RDF iCalendar draft.
The name of this bothers me slightly, as an attendee is usually a person, not an address. In addition, in terms of modelling it conflates a person with their email address.
An RDF schema for the 'core' iCalendar properties and classes is available here. It needs describing in html too.
I don't know what properties and classes should be required and what should not. This could cause problems for simple parsers and for querying.
The date-time format used is not W3C recommended (as per the RSS 1.0 guidelines).