I'm digging around the newly-released SharePoint 2010 SDK.  I'll put up a post in the near future summarising the changes to security, but in the meantime here's one change I noted:    after (-ed) event receivers can now be run synchronously! 

This resolves a common complaint about SharePoint 2007 after-event receivers, which ran asychronously, often resulting in a race condition, invalid data, and confused users.

It's implemented in the code used to bind the event receiver, and not in the event receiver itself. Basically the SPEventReceiverDefinition class includes a Synchronization attribute, which can be set to  SPEventReceiverSynchronization.Synchronous or SPEventReceiverSynchronization.Asynchronous (or, finally, SPEventReceiverSynchronization.Default);

SPEventReceiverDefinition eventReceiver = eventReceivers.Add();
eventReceiver.Synchronization = SPEventReceiverSynchronization.Synchronous;
....

Beffore event receivers are still always run synchronously,as they can be used to cancel the action. If you needed to, you could always spawn a new thread within your synchronous event receiver)