My receiver pack of choice is LiFePO4 (A123).
These packs put out 6.6V which is low enough to avoid damaging receivers or (most) servos and the A123 cells have a more than 30C rating, which means you aren't going to get brownouts, no matter what the load.
For more info on the various battery technologies I suggest you read
this article on the RCModelReviews website.
As for receivers, I know that FrSky are producing an "HV" version of their receivers that will safely run directly from a 2S LiPo and there are also a number of HV servos available from various manufacturers that will also run on 2S LiPo.
However, as I see it, LiPo has too many downsides for use in a receiver/servo pack.
For instance, have you ever left a model accidentally turned on when you put it away and come back to find the batteries were dead flat?
If you were using a LiPo then that battery would be wrecked.
If you were using a LiFePO4/A123 pack then you could just recharge and fly again (I know this because I've left the same model turned on several times and totally flattened the A123 pack in it -- it still has full capacity even after this abuse).