1. Self-contained, untethered synchronization, much the same way a Blackberry gets email.Applications on the Treo like Quick News and Resco Neeews! can manage your RSS feeds and download podcasts to your SD card, and the latest version of Kinoma Player can show a RSS feed as a playlist letting you stream podcasts over-the-air.
2. Read-write, two-way, should be able to record and connect with a publishing system for automatic upload and feed production.
I can think of two ways to podcast from my Treo. First, I can use it to call a system like LiveJournal voice post and just talk into the phone, recording like I would with another cell phone. Second, I can record to a wave file using the built in Voice Recorder or I could record a movie with the Camcorder app. For editing and adding sound effects and music, you can use Bhajis Loops or Wave Edit Pro. A 2GB or 4GB SD card can provide plenty of space for media creation.
3. Must be a platform, that is, people other than the manufacturer can add apps.Palm OS is one of the largest mobile platforms, and there's plenty of applications out there now with more on the way.
So, what are the limitations? First, the audio jack on the Treo is 2.5mm, which means you need an adapter to use good headphones and microphones with the device. Second, the current Treo phones don't have WiFi, so all data transfer is done through your cell provider. This can be OK if you've got unlimited data and fast service, but sucks in places like Europe. Third, you don't get to be a cool kid with a new device, you're just a Treo user finding another way to use mobile computing for your benefit.