AE CS5 64bit - there is a bug when rendering out fields using multi processors. Going from an HDTV 1080_25 preset comp to Animation codec (no scaling - output is 1080i25). Have the output dialogue box set to field render UFF. If you turn multiprocessing on to get the benefits of faster render and preview you get a correctly flagged UFF file but checking the file in AE it is actually progressive (well two identical fields in a row - no movement). Do the same render again with Multiprocessing disabled and the target file is created correctly with fields.
Don't know if this is a known problem but could see no reference to it on the interweb so thought I'd post it here - HTH some other users and hope Adobe addresses it.