years and years of experience with these kinds of things (currently working with "high end" web applications)
my only requirement would be java, anything else you can define
i refuse to create webapps with microsoft products heh
sso (single sign on) with existing systems shoudl be no problem
the backend can be done via webservices that c# integrates into nicely (and is xml which is easy to marshall/unmarshall)
edit: are there any specs for these needed services so that i could look at em and draft some usecases/examples/documentation out of it to see if youre interested in using something else than microsoft technologies