After searching bbforums for an exhaustive post on this topic, and coming up empty-handed, I figured I would take a stab at making one...

T-Mobile currently has three kinds of prepaid plans:

Pay By The Day - Self descriptive.
Pay As You Go - Your standard prepaid plan; refill your account as you go.
FlexPay - What you need if you wish to use a BlackBerry (and all of its features) in conjunction with a prepaid plan.

Why do I need a prepaid plan?

For most cell phone/BlackBerry users a standard plan works just fine and often saves them money in the long run, but a prepaid plan offers freedom from signing an annual (usually two year) contract. It also allows you the freedom to change carriers at any time without having to pay an Early Termination Fee (currently $200 with T-Mobile). Finally, it doesn't require a credit check for the potential account holder.

How does it work?

FlexPay is a fairly simple concept. You pick a plan (most, but not all T-Mobile plans are offered to FlexPay customers), add the features you want (*hint* T-Mobile offers unlimited BlackBerry service for $19.99 with applicable voice plans), and you pay for service at the beginning of each cycle (instead of at the end as with your traditional plan). FlexPay account holders can opt for month-by-month service, or annual service.

Do I have to buy a new phone/BlackBerry?

No. If you already have a device that you are happy with T-Mobile does not require you to purchase a new phone when signing up for a FlexPay account.

Will I get a discount if I want a new phone/BlackBerry?

Opting for the month-by-month FlexPay service does not entitle you to a discounted T-Mobile phone. But a one year service will.

What are the drawbacks?

For users of the 8320, the HotSpot @Home Add-On will not be available.

T-Mobile charges a monthly "Control Charge" of $4.99 to FlexPay customers, but will waive it if you sign up for direct debit aka "EasyPay" (automatic payments of your bill each month *these do not "lock" you into a contract, you can still cancel at any time*).

FlexPay accounts operate off the prepaid network, and like T-Mobile ToGo, may only roam in Canada and Mexico.

Want more info?

The number for T-Mobile's FlexPay department is: 1-877-882-7164

Visit T-Mobile's website for info on plans available to FlexPay holders: FlexPayPlans

I heard about this last week and I think it is great.

We need something like this in the UK!

When I signed up in November, they offered me FlexPay as one of the options although didn't use that name. However, you didn't get the rebate or the discount on the phone. Since that was worth about $175 total compared to a potential $200 termination fee, it wasn't a good option.

But, for someone bringing their own unlocked device, that's a pretty good alternative to a long term contract.