I really like Spark, but we\'ve already got a big investment in webforms based views. I\'d like to start using Spark without having to convert all my existing views up front.
Just an idea, try to write a custom view engine (or tweak Spark's one) that will render the requested .spark view using Spark engine, and then call WebForms engine passing it the rendered content - I think this should be possible, but I have no knowledge about these internals. An easier way would be to save rendered .spark HTML into .aspx and have WebForms pick it - but the performance will not be acceptable.
Anyway, I'd say it should be possible but 1) will require complex tweaking of view engines and 2) you'll lose much of the Spark/layouts functionality since your .spark views will mostly render as without spark layouts at all.
Also, try to ask in Spark mailing list.