Meredith Vieira appeared as herself in last night's episode of '30 Rock', in her duties as a presenter on 'Today'. Here is a list of other appearances by Ms. Vieira's televersion:
'All My Children'
'Spin City' - but that takes place in an altermate TV dimension.
Additionally, the use of the 'Today' show in a fictional way links '30 Rock' to 'Seinfeld', 'The Nanny', 'Will & Grace', and other sitcoms.