The best way to answer your first question is to look in their job listings, or in this case their blog. This is something I learned from PG's story of looking at competitors' job listings in his Viaweb days.
They want (or wanted) someone with "Heavy experience with C# and/or .NET in general". That makes me think Flash isn't the only important technology for them.
Loopt has some .NET in their architecture and Sam Altman told me at startup school that it was never an issue with YC.
To answer the question though, I believe I've heard in a few places that they don't care what you use, as long as you are building something worth funding.