Cookies and subdomains
Yes, you can. Use:
Response.Cookies("UID").Domain = ".myserver.com"
The easiest way to apply a cookie domain that can be shared across subdomains is to put it in your web.config:
<forms cookieDomain="example.com">
Yes, but beware don't set same-named cookies in various subdomains, as the resulting cookie appears to be random; instead, set one cookie in the .maindomain.com only (not in any .sub.domain.com)