I'm Birte from txtr.
As BlueMonkey said, you can also read DRMed content from other sources, but you have to "work" a bit on it. Now we postet a "How To" in our Blog
Originally Posted by BlueMonkey
Thx as well for the credit
In their official blog (German)
they have pointed out that at the moment only DRMed EPUBs and PDFs bought from their own store work right away with the app. For DRMed content from other sources you seem to need a little work around which they promised to explain as soon as possible. I will forward and translate this here as soon as they publish the instructions.
There is one Problem for Americans: The txtr-iPhone app right now needs one book from the txtr Store to be activated for DRM