I think the answer to this question is so obivous that noone has bothered writing about this, but its late and I really can\'t get my head around this.
I use this example from Michael Puleio's blog on using a HttpModule to handle the build up of my dependencies using Unity. http://blogs.msdn.com/mpuleio/archive/2008/07/17/proof-of-concept-a-simple-di-solution-for-asp-net-webforms.aspx