It looks like all of the recent Nexus devices are covered - everything from the 2012 Nexus 7 up through to the Nexus 5. Of course, just because branches have been published for these devices, it is not absolute confirmation that this entire list of devices will receive an official L release. Nothing is certain until Google releases OTAs and/or factory images.
- Nexus 4 - https://android.googlesource.com/device/lge/mako/+/l-preview
- Nexus 5 - https://android.googlesource.com/device/lge/hammerhead/+/l-preview
- Nexus 7 (2012 Wi-Fi) - https://android.googlesource.com/device/asus/grouper/+/l-preview
- Nexus 7 (2012 GSM) - https://android.googlesource.com/device/asus/tilapia/+/l-preview
- Nexus 7 (2013 Wi-Fi) - https://android.googlesource.com/device/asus/flo/+/l-preview
- Nexus 7 (2013 LTE) - https://android.googlesource.com/device/asus/deb/+/l-preview
- Nexus 10 - https://android.googlesource.com/device/samsung/manta/+/l-preview
Note: there still aren't any binary packages or kernel sources for the Nexus devices, but these are probably coming fairly soon.
It's not clear yet if all of the source code is fully uploaded, but I've yet to see a project that doesn't include an "l-preview" branch that appears to contain a full file structure.
Update: It turns out this wasn't a complete release of the L preview source code, after all. Bill Yi posted on Google Groups to point out that this was an update for some GPL projects, but it's not a complete platform update. Subsequent comments revealed that an l-preview branch was created for many non-GPL licensed projects, but the contents of those branches match the current state of their related Master branch. There's still a lot to be taken from this update, but full custom builds won't be possible yet. At least this is pretty strong evidence of which Nexus devices will receive updates to the final L release.
It's time to start poking around to see what we can find!