Keep in mind I'm running this via localhost on a Vista PC so IIS 7.  I've tried both using just plain localhost as well as the VS web server for localhost:port. 

here's the request /  reponse being sent over the wire:

 POST /xxxxxx.aspx HTTP/1.1
Host: localhost
User-Agent: Mozilla/5.0 (Windows; U; Windows NT 6.0; en-US; rv:1.9.0.5) Gecko/2008120122 Firefox/3.0.5 (.NET CLR 3.5.30729)
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Language: en-us,en;q=0.5
Accept-Encoding: gzip,deflate
Accept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.7
Keep-Alive: 300
Connection: keep-alive
Referer: http://localhost/xxxxx.aspx
Content-Type: application/x-www-form-urlencoded
Content-Length: 249
__VIEWSTATE=%2FwEPDwUKLTEwNjg4NTM5OGQYAQUeX19Db250cm9sc1JlcXVpcmVQb3N0QmFja0tleV9fFgEFDmJ0bkxvZ2luQnV0dG9uejCq%2FmxnC032x2Pvh7Yu8vXJwm4%3D&__EVENTVALIDATION=%2FwEWAgKj8tnxBAKR2da8DAJkTWCYFwyhYjDbsPvNqaUBw%2BY5&btnLoginButton.x=41&btnLoginButton.y=14

HTTP/1.x 500 Internal Server Error
Cache-Control: private
Content-Type: text/html; charset=utf-8
Server: Microsoft-IIS/7.0
X-AspNet-Version: 2.0.50727
X-Powered-By: ASP.NET
Date: Thu, 22 Jan 2009 17:38:41 GMT
Content-Length: 5901