Thanks for pointing that out. It looks like there was an error in the previously uploaded version (2.3.3) which only manifested itself when imported into a new project. We're working on a fix and will upload a new version.
Update - turns out there isn't an error, but an additional case that needs to be handled somehow. This only happens when a Unity project is created with the "2D" default settings, and then importing tk2d. Because tk2d is backwards compatible to Unity 3.5, there are some missing values for settings in there, namely "Alpha Is Transparency". Normally that would be fine, but Unity picks different default values when running in "2D" or "3D" mode, and as a result in 2D mode it wrongly ticks Alpha is Transparency. I'll have to investigate how to detect and change that, as it is a setting that Unity sets on the texture as soon as its imported, before we get any control over it.