It is very difficult to answer how many developers participated in this project. It was used some independent consultancy, suggestions, recommendation etc. Also It was used some open source code. There were a lot of brilliant ideas on this forum. So it was done rather by a joint effort and a special thanks to all these people.
Regarding Silverlight 5 it is very early to plan something. The final release is planned for the end of 2011.
Currently we have some active tasks and bugs with current version of Silverlight. Also it is being developed version of SilveOS for Windows Phone 7. It will be available in the beginning of next year.