From an nntp posting:

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Queries to obtain the caller's name based on the calling party number. The name database is administered by the local exchange carrier providing the calling name service. Some smaller telco's may share a
name database, or lease one from another provider. The name database is most likely populated from business (billing records). New lines or recently moved lines will take some time to show up, as this is a separate DB from the actual telephone office "data" and is also (likely) separate from the real billing DB.

There is also a possibility that access to the name database is unavailable or that the DB will take "too long" to respond with the information. So the call is completed with out the callers name. Such occurrences should be very rare.
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Hmmmm, in the case of T-Mobile to an SBC landline, I'm assuming T-Mobile routes the call via their network but since it's routing via Cingular (e.g T-Mobile is a VNO in California/Nevada using Cingular). SBC picks it up and is using some stale name lookup database.

Thing to try is call someone else who is on a different landline provider (e.g. in a different market), like with Verizon landline or Qwest and has caller ID with a phone that has name listing. See what shows up.