once you jailbrake your phone can you still access the standard at&t app store? or does it eliminate all the regualr apps and only allow cydia apps?

also how can you change the spring board is it done with an app?

and lastly if my phone is tethering now on 3.0 and with vmm and mms once i jail brake will i have to resetup the tethering? or will it follow the jailbrake?

Yes apps from cydia sit happily side by side with apps from AppStore on a jailbroken phone. AppStore still works fine after jailbreak. Alsk a small correctioon, AppStore is owned and run by Apple not AT&T

Springboard changes can be done in many different ways. One of the most popular is winterboard. I don't use this at all so will leave this question to be answered by someone else.

If you use redsn0w to jailbreak your existing 3.0 everything you have configured regarding mms and tethering will still work. If you ever restore your OS to 3.1 you may need to jailbreak to get tethering if your carrier does not officially support it on the iPhone.

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iPhone 3GS 32GB factory unlocked (3.1; Pwnagetool, Cydia, OpenSSH)
iPhone 8BG (3.1.2; Pwnagetool, Cydia, OpenSSH)

thank you, ive been trying to solve that question for months now.

and i would be upgrading to 3.1.2 and jail breaking that and using the mywi for tething once get there.