Thanks to Ian Billings we now have some new information and actual images of the new security (slit) Machins due to be issued this month.
His scans are really cool, showing all aspects of the security features, these can be enhanced if you click on the pics which show the features in great detail.
Ian also posts some of the printing dates that he has seen. These are reproduced below (Thanks Ian)
1st class - 03/12/08
2nd class - 04/12/08
50p & £1 - 08/12/08
1st Large - 09/12/08
2nd Large - 10/12/08