i was having the same exact problem, this is what i did to fix it...
SYMPTOMS
After you type your credentials and then successfully log on to Microsoft Outlook Web Access (OWA), you may be prompted for your credentials again.
CAUSE
This problem occurs because a default domain is not specified for Basic Authentication in the Public virtual directory. By default, the Public virtual directory is located under the Default Web Site in Internet Services Manager.
RESOLUTION
To resolve this problem, specify the default domain in Exchange System Manager as follows:
1. Click Start, point to Programs, point to Microsoft Exchange, and then click System Manager.
2. Under Servers, expand the Protocols, the HTTP, and the Exchange Virtual Server nodes.
3.Right-click the Public virtual directory, and then click Properties.
4.Click the Access tab, and then click Authentication.
5. Under Basic Authentication, type the same value in the Default Domain box as the default domain for the Exchange virtual directory.
6. Click OK two times.
7. Right-click Default Web Site, and then click Stop.
8. Right-click Default Web Site, and then click Start.
also you can check out the thread i posted a few weeks ago about this...
let me know how you make out, good luck!