Please be sure to check out the team blog we have here
This has the most input from the broader team on profiling and performance issues. I'll talk somewhat in my blog about some of the technology we use, but that site is always your best bet.
For the moment, I'd like to solicit feedback about when and where people are using profilers to figure out performance problems. Is it mostly in the field, after apps have been deployed, or before deployment on the devs desktop? What issues do you encounter when you try to profile in those scenarios with existing tools?