<p>I could really do with updating a user's session variables from within my HTTPModule, but from what I can see, it isn't possible.</p>

<p><em>UPDATE:</em> My code is currently running inside the <code>OnBeginRequest ()</code> event handler.</p>

<p><em>UPDATE:</em> Following advice received so far, I tried adding this to the <code>Init ()</code> routine in my HTTPModule:</p>

<p><code>AddHandler context.PreRequestHandlerExecute, AddressOf OnPreRequestHandlerExecute</code></p>

<p>But in my <code>OnPreRequestHandlerExecute</code> routine, the session state is still unavailable!</p>

<p>Thanks, and apologies if I'm missing something!</p>

HttpContext.Current.Session
should Just Work, assuming your HTTP Module isn't handling any
pipeline events
that occur prior to the session state being initialized...
EDIT, after clarification in comments: when handling the
BeginRequest event
, the Session object will indeed still be null/Nothing, as it hasn't been initialized by the ASP.NET runtime yet. To work around this, move your handling code to an event that occurs after
PostAcquireRequestState
-- I like
PreRequestHandlerExecute
for that myself, as all low-level work is pretty much done at this stage, but you still pre-empt any normal processing.