Sportablet is an Android app that allows you to connect many Garmin devices to Android tablets. Primarily intended for fitness buffs, it gives access to stats, allows you to post to Garmin Connect or Facebook and more. While it’s not designed for Garmin handhelds, it does appear to work with many of their more recent introductions. There is also a smartphone version – the Uploader for Garmin.
Data transfer is one-way for the time being, but it looks like sending to the Garmin is on the development roadmap for a few units. According to the developer, it’s a pretty tough row to hoe though, and the Edge series is the most likely to see it first. Too bad it’s so hard to do; I know lots of geocachers who would love to be able to grab a pocket query on the fly and transfer it to their handheld. And wouldn’t it be nice to be able to download a track from Garmin Connect at the trailhead for backcountry navigation?
The developer has told me he is always interested in supporting new devices, so if yours doesn’t work consider getting in touch, especially if you’re willing to beta test.