Bridgehunting with your GPS navigator
I've uploaded a ZIP file (1.8 MB) containing the necessary POI information for Garmin and possibly other GPS receivers. Inside is bridges.gpi, which can be loaded directly to recent Garmin models, and bridges.gpx, an XML file that can be edited and converted for other receivers.
Here's how it works for Garmin devices
(Note: These directions worked for me, but your mileage my vary. If you brick your device, you're on your own.)
Garmin makes a POILoader.exe program for loading custom data, but it doesn't run on my Linux system. However, my Garmin (and remember this is a low-end model) has an SD card reader, so I can copy the garmin.gpi file to the card and insert the card into the GPS.
After turning it on, the Garmin shows this message: "New Extras data found on card. Would you like to install this data so that it will be available if the card is removed?"
If you tap Yes, the entire bridge database will be permanently loaded. Removing or updating the data will require connecting the unit to your computer with the correct USB cable. If you don't have that, I'd recommend tapping No so that the database isn't permanently loaded -- you'll still have access to it from the SD card. Despite weighing in at a hefty 7 MB, the database seems to fit comfortably within the internal storage space.
Next, you can access the bridge data by going to Where to -> Extras -> Custom POIs. (This might vary on higher-end models.) You will see a list of the bridges that are closest to your current location. If you tap one, you can see a summary for the bridge.
I've included a fair amount of information about each structure, including the name, Bridgehunter ID number (to make it easy to search the site for a particular bridge), overview, county, status, and history. You can tap the "More" button to see the whole summary in an easier-to-read format, although (on my Garmin at least) the summary is repeated twice for some reason.
The bridge data does not show up in the regular Points of Interest search. To look for bridges far away from you, the only option is to select a new locaton manually and then go to Custom POIs to see those bridges.
Updating or removing the database
To make changes to the installed data, you will need a mini-USB cable to connect the device to your computer. Your operating system should treat the Garmin as an external storage device. To remove the data, go to the /Garmin/POI folder and delete the bridges.gpi file. Copying over a new version from your computer doesn't seem to work (at least in my testing). It may be possible to copy or update using the POILoader.exe program, although I haven't had the chance to try that.
Adding bridges
If, while travelling, you stumble across an interesting bridge, you can easily save the coordinates for later. Go to Where to? -> Coordinates -> Next -> Save -> (Enter a label for future reference) -> Done. This will add the current location to your "Favorites."
When you get home, connect the device to your computer and look for the /GPX/current.gpx file. This contains an XML listing of your favorite places, which you can view using a text editor to extract the latitude/longitude coordinates.
(Warning: In the process of fiddling with my Garmin to write these instructions, I somehow loaded a bunch of random bridges from the database into my Favorites. I didn't lose any of my existing Favorites, but now I have a bunch of bridges from who-knows-where cluttering the list, with no easy way to remove them. So be careful.)
Other brands
The ZIP file also contains a bridges.gpx file using the standard GPX format for storing GPS information. It might be possible to use gpsbabel or another program to convert this file into the POI format used by TomTom or other GPS brands. I believe it's also possible to convert GPX into the KML format used by Google Earth.