Actually the iPhone / iTouches will play 24 bit ALAC files. They're just limited to 24/48 max. Not sure about the iPods though - would be surprised if they weren't the same though.
Anyway - OP - easiest way is to transcode a copy of your files. I use dbpoweramp. It allows you to resample, and also reduce the bit-rate. I would encourage you to actually try a blind test some time between a high-res 24/96 (or even normal flac 16/44.1) file, and a transcoded aac256 file from the same source. Google "foobar2000 abx". You'll probably learn that you won't be able to tell the difference when transcoded properly. Then you can keep your lossless for home, but use a fully transcoded aac256 library for your portable - and safely know that you're not missing out with either