When reading a mail, text can be selected (by "double-touch-drag"), but not copied. Tested with standard Strg+C on N900. It should be at least possible to copy text from a mail that is currently read to a mail that is currently written. Observed on 4:4.5~20100902.1170752-1maemo1.1170522 .
Available now with Actions -> View -> Copy Email To Clipboard.