The latest JOSM now supports multiple data layers again. This was requested since about the day after I removed the feature over a year ago ;-).
As another big data source hits the main database (AND’s netherland-parts-of-china-and-india map) the feature got more urgent. I think it is especally cool for people who want to compare two unrelated datasets ;-).
Every file open and every download opens the data into a new, seperate layer now. You can merge these layers using the merge button in the layer dialog. By selecting an layer in the layer dialog, you can switch the current editing dataset as needed.
Now for the bad news:
As the feature is a ground shaking one for the whole JOSM data holding structure, I expect many new bugs introduced :-(. If you run into problems that block you from mapping, please revert to the latest release from last week.
For Plugin writers:
Plugins, that used Main.ds could be affected as well. Main.ds now holds the dataset of the current editing layer rather than all the data together. If you are a plugin writer and your plugin is broken and you cannot fix it to work with josm-latest *and* the josm-1.5 release, please link a version which is working with the latest release at http://josm.openstreetmap.de/wiki/Plugins/LatestRelease.