I purchased a Macbook Pro about 6 weeks ago. Its been great for the most part. Rather than immediately putting Kubuntu on it, I've been forcing myself to learn OSX. I like it, but its not all that much different than my Linux user experience has been. In fact, when it comes to USB audio devices, its been worse. Linux was 100% plug and play for all of my audio devices. Two of which are my Belkin TuneStudio and my SoundBlaster Extigy. I use the TuneStudio  to record podcasts on Skype. It works awesome! Just plug in your iPod and you have a very reliable, simple, almost pro quality device for recording with your computer. The Extigy is just a play awesome usb device. Even though its old it just rocks. Lots of digital options and very high quality sound.

On my Macbook, the TuneStudio worked, but the Extigy didn't. I tried one fix found at this thread on the Apple Support forums. Basically you replace the AppleUSBAudio.kext file with an older version. But it didn't seem to work and, in fact, made the problem worse because now both the TuneStudio and the Extigy didn't work. I kept looking and found a related post that also replaces the IOAudioFamily.kext file as well. Once I did that everything worked. I didn't even need a reboot. Here are the steps that I took.

1. Download the 10.5.6 versions of AppleUSBAudio.kext and IOAudioFamily.kext to your download directory.
2. Backup the current versions of those files from /System/Library/Extensions. If something goes wrong then you can always go back to the original versions. At the very least make sure you have a good Time Machine backup if you don't specifically back up those files.
3. Now copy the 10.5.6 versions into the /System/Library/Extensions directory.
4. You make get a few errors that pop up about those files being install improperly. Don't worry! They just need the right permissions set. The easiest way to do this is to open the Disk Utility and select the volume that your OSX is installed on (Macintosh HD is the usual one). Then click on the Repair Disk Permissions button and let it do its thing. It should list the files you replaced as files that it repaired in the details log.
5. Plug in your USB Audio devices and they should now appear properly.
6. Swear at Steve Jobs for making you do all this extra work!

Or in my case, wish for the newest version of Ubuntu to get released so I can install it on my Mac. I decided to wait for the newest version to force me to use the Mac OS and for the knowledge that Karmic will have even better direct support for Mac hardware in it.