This should be possible with a URL rewrite rule similar to this:
http://weblogs.asp.net/owscott/archive/2010/01/26/iis-url-rewrite-hosting-multiple-domains-under-one-site.aspx
<rule name="site1.com" stopProcessing="true">
<match url=".*" />
<conditions>
<add input="{HTTP_HOST}" pattern="^(www.)?site1.com" />
<add input="{PATH_INFO}" pattern="^/site1/" negate="true" />
</conditions>
<action type="Rewrite" url="\site1\{R:0}" />
</rule>
Jeff Graves
OrcsWeb: Managed Windows Hosting Solutions"Remarkable Service. Remarkable Support."