From Halo forums:
'We are on Sky. Their new broadband shield feature was blocking the site. I logged in to MySky on the Sky website and was able to update the broadband shield settings to add halowaypoint.com as an allowed site. The multiplayer now works! Hurray! If you are not on Sky then maybe your ISP has a similar Internet protection feature that you can update to allow your xbox to connect to halowaypoint.com'
Or better still turn the shield off altogether, it shouldn't be necessary if you've got anti virus software anyway, and it's likely to only cause further problems.