Not sure about Nextel but Cingular does this also. I have noticed it when I have lost total connectivity to a cell and it re registers. I have not seen any issues with getting email after but it wont receive any messages if the BB is not reconized by the system. I do see this only at my house and I have poor reception at that location.
One other thing may be your plan. you indicate you dont have a BBplan. maybe Nextel has issues with the setup you have.
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Z10 on BES