Yes, a respectable line-in would be really nice. For now I have been using the Wolfson Audio Card but it costs as much as the Raspberry Pi itself, and you have to patch the OS to get it to work (but the quality is great). I've tried some of the USB audio devices but found the quality to be not good enough, probably because they are really designed for microphones and even with gain at minimal settings there is too much distortion for line-in. 48kHz is good enough IMO, the Pi doesn't really have enough CPU power to do much more than that if recording and compressing in real time.