When selecting a folder, mails cannot directly be deleted by a press to the delete icon. The button becomes "active" however, after viewing a mail for the first time in the folder. Observed in version 4:4.5~20100827.1168757-1maemo1.1169941 .
This has been fixed quite some time ago already.
Confirmed, version 4:4.5~20100928.1180427-1maemo1.1180334 .