It's a great time to be a managed code developer! Our fearless leader, Jason Zander, has just blogged about our plans to enable the frequently requested feature of in-proc SxS CLR's. To date, it has been difficult for me to respond (i.e., not comment) to inquiries from ISV's about our plans to enable SxS CLR's in process. I suppose it may have helped my poker skills.
The work we have done on Silverlight has included architectural enhancements to enable the "full" CLR and BCL(s) to run SxS in process. This functionality further strengthens our upcoming extensibility offering of the Orcas Add-In BCL.