I hope I've solved your problem with iTunes Plus. I've found that there is a change in the structure in the .m4a files between QuickTime/Nokia/everybody else and the ones from the iTunes Store.
It has a simple fix (better than re-wrapping via QuickTime) that I have put on my blog but it still means you have to run a tool over the file. However mine doesn't destroy any metadata (works on all my 27 iTunes Plus .au tracks). And they work now on my E61.