Huh. According to the 'About 3G' tab on the T-Mobile G1 Website, the fine print says:
If your total data usage in any billing cycle is more than 1GB, your data throughput for the remainder of that cycle may be reduced to 50 kbps or less. Your data session, plan, or service may be suspended, terminated, or restricted for significant roaming or if you use your service in a way that interferes with our network or ability to provide quality service to other users.
1 GB is certainly a lot of data to use per month but it isn't an unreachable, 'unlimited' amount of data. Especially with Google Maps Street View and the T-Mobile G1's tie-in with all things in the Google Universe (which all require Internet) we don't know how taxing it all will be on T-Mobile's network and what kind of data it needs.
Even if we won't reach the 1 GB threshold, we at AndroidCentral think that T-Mobile should bump up the "unlimited" amount of data because if they say unlimited, we want unlimited.