I seem to have solved my own problem. You can't access the Request object at Application_Start with IIS7.0, though you can use it in a custom route constraint. Here's how I did it.
Define the custom route constraint:
Imports System.Web
Imports System.Web.Routing
Public Class ConstraintHost
Implements IRouteConstraint
Private _value As String
Sub New(ByVal value As String)
_value = value
End Sub
Public Function Match(ByVal httpContext As System.Web.HttpContextBase, ByVal route As System.Web.Routing.Route, ByVal parameterName As String, ByVal values As System.Web.Routing.RouteValueDictionary, ByVal routeDirection As System.Web.Routing.RouteDirection) As Boolean Implements System.Web.Routing.IRouteConstraint.Match
Dim hostURL = httpContext.Request.Url.Host.ToString()
Return hostURL.IndexOf(_value, StringComparison.OrdinalIgnoreCase) >= 0
End Function
End Class
Then define the route:
routes.MapPageRoute(
"Search_Area1",
"Search",
"~/area1/search.aspx",
True,
Nothing,
New RouteValueDictionary(New With {.ArbitraryParamName = New ConstraintHost("domain1.com")})
)
routes.MapPageRoute(
"Search_Area2",
"Search",
"~/area2/search.aspx")
)
This technique can also be used for applying different routing based on the sub-domain as well.
Big thanks to Steven Wather's asp.net mvc routing post for pointing me in the right direction (even though it was for mvc and not web forms).