Hello,
i bought me a new iPhone.
Now I am waiting for the parcel
I want to use this phone with my non-tmobile provider sim.
I dont know the baseband Version.
Are I am right?
Jailbreak = run unsignet software BUT still locked to t-mobile?
Yellowsn0w= Software unlock for all provider? But not with the new Baseband?
Is it possible to downgrade the Baseband?
Or is the only possibility a turbo sim card adapter?

Yes

Yes

Yes, if you have Bootloader 5.8 which is VERY rare. So I would say NO

Not reliable. Don't work with 3G.

Well I'm currently on a turbo sim adapter because I accidentally "upgraded" to 2.2.1 the day I got my iPhone. Anyways, it works decently. It's the only temporary option, other than using another phone while carrying around your iPhone as an iPod Touch.

The one thing about some of the adapters is that once your iPhone completely goes to "No Service," you have to remove the actual SIM card and place it in another T-Mobile phone to regain signal, then you can put it back in your iPhone and it'll work fine.

Something usefull to have if you're going with the adapter is install SBSettings and "Show" Airline mode, so if you're going to be somewhere that you KNOW you won't get service, you won't have to do the SIM re-activating. Schools, hospitals, tunnels...a lot of the obvious areas.