T-Mobile is capping UMTS network use at 1 Gigabyte. When you read the fine print ad the bottom of the G1 3G description pages it states:
"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.Some devices require specific data plans; if you do not have the right plan for
your device, you may not be able to use data services. Some downloads, such as movies,
music, and games, not included. Domestic use only.
"
Once you are demoted to 50 kbps that means you are off 3G speeds.
T-Mobile G1 $25 data plan offers 400 messages and unlimited
data access and the $35 plan adds unlimited messaging. A few hours later the cap was removed due to "PR" pressure.