I updated Winamp today from the market and now I'm having troubles playing shoutcast stations and some mp3's. Is it possible to rollback only the update I installed this morning? or am I out of luck?
no, you are pretty much out of luck. Once you update, you have that version and cannot go back.
I do not know your android knowledge, but, if you have the apk file for a previous version, you can "sideload" the application on to the device (unless you have at&t). But that is a bit more complicated if you never done it before. It requires the drivers for your PC, copy the file to the sdcard, and then "open" the apk file with an file manager, which will then launch the Package Installer.
I guess it will be fixed in a future update but until then, have you tried contacting the developers for an older version. I'm sure they'll be appreciative of any feedback you give and it would be easy for them to provide you with a previous version directly.
If you are feeling brave, I found the following two sites that might have what you need as a direct download, may need something like Astro file manager (free) to install and will need to enable unknown sources in the settings.
I don't know how reputable the sites are so I would try my first suggestion initially.
For free apps, you can typically find an older version on ApkMirror. They keep older versions of most popular free apps and have good reputation of being malware-free.
For paid apps, you'll have to use your own backup (it's always a good idea to make backups of your data from time to time, including the apps you're using), or look at more dodgy websites. In the latter case, it's highly recommended to check the APK with VirusTotal before installing it on your device.