Rivian will let preorder holders take delivery "in the next 6-8 weeks or sooner".

Downside is, you'll have to drop your preordered model for any of the available models already built and listed in the 'R1 Shop'. This means you might not get an R1T exactly how you like it and will have to settle for whatever shows up in the shop. But for the impatient, this might be worth a much quicker delivery.

Rivian recently sent out an email about the newly launched shop that can be found here:

Access to the shop is only available through an invite that Rivian has already started sending out to preorder holders via email.

Important to note is that Rivian will just give you a week to decide if to go with a model from the R1 Shop or not, according to this statement at the end of the email:
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At the moment the shop lists basic vehicle configuration info and won't give you a VIN or anything specific.