If you can wait until later this year there's a Haswell variant rated at 4.5 SDP coming out later in limited quantities. It'll allow for fanless tablets. Hawell Y-series at 6 SDP still requires at least some active cooling.
However, I'd expect them to command a price premium, and top end performance is likely to be thermally limited as SDP is mostly relevant for light workloads (like what you generally see on tablets). It should do even better than the Haswell ULT in the Apple MacBook air (web browsing bench at Anandtech, 14.28 hours with a 54 Wh battery, 11.24 hours normalized to a 42.5 Wh battery like the iPad 4).
But is unlikely to be able to handle higher workloads as well (heavy PC applications) as an actively cooled Haswell solution. As both 4.5 SDP and 6 SDP Haswell variants have the same 11.5 TDP (higher workload applications than what is used to rate SDP).
Regards,
SB