So I think Maria Hill (Colbie Smulders) will appear in the Agents of SHIELD pilot, possibly as a precursor to her regularly guest appearing or even becoming a regular cast member after HIMYM ends in 2014 (which would leave her able to join as a regular cast member in season 2 of AoS).
There's some evidence in the trailer for the show that this will be the case. After the opening voice over (who I believe is Skye), with the shots of the city, the shots of Cap's shield, Thor catching his hammer, etc, and the Marvel logo, we then hear a voice ask (at about the 0:25 second mark) "What does SHIELD stand for Agent Ward?" We get his snarky reply and then "What does that mean to you?" from the woman interviewing Ward (who we conveniently never see) before Coulson walks in and introduces himself.
Listen to her voice closely. Any HIMYM fans could tell this is most likely Colbie Smulders--her voice is pretty unique, even more so in Avengers with the more hard-edged and forceful delivery she gives.
But seriously, watch the trailer again and tell me that's not her. Think we'll see Maria Hill in the pilot, likely as just a sort of cameo and likely in that scene only before Ward is passed off to Coulson.