Garmin released their Android-powered smartphone that has Garmin navigation tools built into a cell phone. I don't see any indication of geocaching features on it, but with an Android phone you could load the standard Geocaching apps, right? And if Garmin has entered the Android phone market now, I wouldn't be surprised to see a version of the iPhone app ported over for Android.

http://www.garminasus.com/en_US/

Here's the text of the e-mail I got:

Introducing Garminfone, the first Android™-powered smartphone fully-integrated with Garmin’s premium navigation experience. With Garminfone, your email, contacts, and calendar content is synced wirelessly – so you’ll always have the right information in the right place at the right time.
Garminfone also offers users a rich mobile Web experience with integrated Google™ mobile services including Google Search by voice, Google Maps™, YouTube® and access to the Android Market, where you’ll find thousands of applications to make the most of this full-featured smartphone.
Visit Garminfone.com to find out more about Garminfone and how it connects your worlds.
It's available for purchase now only from T-Mobile.

I'm finally upgrading from a dumbphone and I'll be testing out the EVO 4G later this week.
I am a big proponent of avoiding multi-purpose items as the quality tends to drop. (For instance, I hate TVs with built-in VCRs and DVDs.) I'll keep my Garmin for geocaching but a spare device for paperless will definitely come in handy.

For me, I think that the nice thing about smartphones with GPS will be that even if I have left my GPSr at home, I can grab a quick couple caches if I have a spare 15 minutes or whatever in whatever location I happen to be in.