You can get it now. Tried signing up, and it went through up to the credit check page (which I didn't finish).
Some clarification now that T-Mobile's site has been updated.
$0-down, 0% APR is only valid if you have monthly On-Demand Data on the Simple Choice Plan. If you're already a current voice customer, you can get On-Demand Data for free. Non-voice customers will have to pay at least $10/mo if they want to take advantage of the Equipment Installment Plan (EIP). If you're already on AT&T Mobile Share and just wanted to go T-Mobile for the EIP, I suggest you just stick with AT&T Next. If you're not a T-Mobile postpaid customer, there's effectively no difference between T-Mobile's EIP and the AT&T Next Program aside from the installment length and monthly payments. Non-postpaid customers will effectively be paying T-Mobile $240 over the course of 24 months for the $0-down, 0% APR EIP. Totally not worth it in my book.
It seems like the EIP is not available on prepaid and T-Mobile is even charging an extra $10 for the SIM card kit if you buy from them. They don't seem to have stock either.
For those who just want the 200MB free data, I suggest buying the Verizon iPad from the Apple Store and just ordering a SIM card from T-Mobile.
yup this is true. I knew it was too good to be true. Pretty sweet though for tmobile voice customers.