is it possible to jailbreak a nintendo DSi or an iPod 5th generation nano
For the Nintendo stuff it's called homebrew. And while possible under the DSi you can only access it in DS mode ATM (no camera, etc).
I've always bought phones from my carrier, so how would I go about getting a Nexus S with T-Mobile as my service provider?
Once again, why would you want to buy it through the service provider?
I don't know how it is in the US but here in Sweden they always add to the monthly subscription cost for a while (usually two years) and lock the phone to them, so you have to wait about two years and then pay them about 500 SEK for the unlocking. So it isn't really possible to take the phone with you if you want to switch carrier.
However, by buying the phone directly from the store, while the "introduction" price might be higher (as you pay for the phone at once), they don't add to the monthly cost, don't force you to use a special subscription and you are free to switch carrier as you please.