Probably one of the biggest shifts in the SCCM 2012 Heiarchy is the requirement for two-way trusts when supporting other domains and/or forests. This can have major implications for those companies who employ a DMZ domain which is not trusted by the internal domain for security reasons.
I came across this blog post about a way to get around this. Please note that this is NOT supported by Microsoft! As I have yet to test this myself, I cannot vouch for its accuracy so your mileage may vary. Let me know in the comments below if you are able to get this working in your environment.