SwirlyScreen ver 2.1 has just been released and it is compatible with iOS4.

The Cydget framework is not yet fully compatible with iOS4 and therefore it is not possible to edit the Lock Cydget Order in the normal way on iOS4. After installing SwirlyScreen 2.1 you can decide which lock screen shall be on top by switching to it (with the home button). When you have the desired lock screen visible, just slide-to-unlock your iPhone and it will be on top further on.

As soon as the Cydget framework is made fully compatible with iOS4 everything will be back to normal regarding this :)

Read more about SwirlyScreen here.