Where are you seeing the bitrate in iTunes?IME, iTunes is very good at displaying accurate bitrates for properly-encoded files (i.e. files with complete, intact headers) in the "Bit Rate" column, under both OS X and Windows - I've never seen it change a bitrate value before. What program are you using to encode from ALAC to AAC?Also, where are you seeing a bitrate value on your iPod? I've never seen an iPod display a bitrate, nor my iPhone 4 (nor do I care ).
I've seen iTunes (for Windows) change the displayed* bitrate on files after having it edit tags, so I don't believe this is at all unusual.(*) right-click on the columns header, select Bit Rate.
what is the latest version of itunes?