EGTRRA 641© expands this to include 403(B) and 457. My question is, who is the "administrator" under a typical 403(B) salary deferral arrangement? In other words, who will be responsible for this - the employer, or the insurance company, mutual fund, etc.? Thanks.

If it's a plan, it has an administrator. If it's not a plan, 402(f) by its terms does not apply. Who actually does it is open, but usually it's whoever gives out claim forms, which is usually the sponsor. The form of notice itself is usually provided by the insurer, TPA, or someone else with specialized knowledge.

According to Reg. 1.403(B)-2 Q&A 3 and the instructions to the Form 1099-R, the notice must be given by the payor. It would seem that this would usually be the mutual fund or insurance company that is holding the account or annuity and that will issue the Form 1099-R.