Does it cost money when i have an "E" on top of my tmobile phone?

[daze]

I dont have a data plan for my tmobile G1 so i searched and found a way to receive mms messages through this site. http://hydtechblog.com/2009/02/06/how-to-get-mms-picture-messaging-working-on-a-t-mobile-g1-without-the-data-plan/ . It worked and i was able to download pic. messages and go on the internet and marketplace. The "E" showed up on top and i checked my tmobile account if i was incurring data charges and it said i didn't so i wanted to know if this costs money because i dont have a data plan or tzones and i am able to go on the internet. Thanks