However, you're going to have to wait a little while longer before you actually can start putting SSRS and SQLExpress together. Mike Wachal (a SQLExpress PM) and Brad Syputa got into a conversation about same, and here are the take-aways:
"The feature: "Using other editions of SQL Server for report data sources and/or the report server database" is not supported in this edition of Reporting Services."
....a few weeks later....
Here are some more details about what Express Edition with Advanced Services will / won't do vis-a-vis SSRS (Thanks to Heidi Steen!):