You have two activations, so there shouldn't be a problem. However, you should probably contact sales@unity3d.com and ask them to remove your old activation.
When you pre-order Unity 3, you should get the serial within a few minutes; shouldn't take longer than a few hours.
Also, you should be getting the download link for the beta within 24 hours, see also: The first Unity 3.0 pre-purchase betas are now shipping! (this links directly to Ethan's posting where he announced that now everyone should have their emails).