Java Joe passed this along via the FLTC email list:
For those of you that own Garmin GPS units, whether “automotive” (such as Nuvi) or “field” (such as 60CSx, Oregon, and Dakota), you should be aware of Garmin WebUpdater, which will update the software (but not maps) on your GPS unit. It is easy to use and particularly important for newer models (such as Oregon and Dakota), as updating may prevent shutdowns and other problems. WebUpdater automatically recognizes your GPS unit once you connect it to the USB port on your computer.
I have a touch screen Oregon 400T, and updated the software with WebUpdater and find that GPS accuracy has improved. What was sometimes +/- 100 feet now seems to be +/- 40 feet at the most; usually better.
How to install WebUpdater? Click this hot link: http://www8.garmin.com/products/webupdater/howtoinstall.jsp.
P.S. Although the touch screen Oregon and Dakota units are easier to learn how to use than the very popular 60CSx, they have no connector for an external amplified antenna, which when mounted high (like on hat or pole), gives the best accuracy.
Henrietta added: Fellow Mac people: Garmin also has a Macintosh version of WebUpdater that works great for newer GPS units. I’ve used it to update my Nuvi car GPS successfully. However, it will not work with older units with a serial interface that you connect to your Mac with a USB-serial adapter. You will need to find a friend who has a PC with a serial port. (My husband was able to update my old Legend using WebUpdater on his work PC.)
New York Outdoors Blog 
I really like what you wrote here. Thanks! I have been experimenting an ipod Nano lately for my geocaching. Have you ever tried that? It’s pretty cool actually, and works well. Visit my site if you’d like to read more.
I needed this, thanks for sharing this information, the Garmin is a good GPS program.
glad you found it helpful – Happy Holidays, Sue
The Garmin Oregon 400t GPS is a great device. I’ve written a short writeup of my experiences and how to use it with some really great free software here:
http://jonathancamp.com/2009/11/16/garmin-gps-oregon-400t/
I’ve outlined four great free resources and website that enable you to pre-plot routes, show tracks travelled and upload / download GPX files easily to your Garmin Oregon 400t.
http://www.gpsvisualizer.com is a particularly great web site with a whole host of free features that making walking / treking offroad easy and a joy.
Interesting … I bought my Oregon 550t pretty recently, and haven’t bothered looking for an update, but it seems like the time has come.
By the way, since you mentioned that the accuracy is improved, but that an external antenna can’t be used to further improve the accuracy – I wonder if you’ve used the waypoint averaging program? It has plenty of obvious drawbacks, mainly that it does nothing for your track log. Honestly I haven’t found much of a reason to use the waypoint averaging, so I’m curious if others have, especially others who are interested in accuracy.
Garmin is nice GPS, but I prefer internal iPhone GPS nowadays.