I posted this on another forum, and thought it might help some fellow FWers here as well...
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Wow... so many different answers to the same questions my head is starting to hurt! 
Personally, I won't be doing this "deal" since I actually LIKE T-Mobile (for the most part) and have $9.99 Unlimited Family Messaging (which works out to $2/line for me)
, but let me see if I can help set some things straight...
1) As of now, with T-Mobile, messaging charges are MANDATORY (ie: text, picture, video, IM). A customer can choose NOT to send any text messages, picture messages, video messages, or instant messages, but currently CANNOT choose who can or cannot send messages to him/her. Since T-Mobile charges for all INCOMING and outgoing messages, and currently has no effective way of disabling (or not charging) customers for inherent network messaging features, customers are charged regardless if the messages were solicited or not.
However, T-Mobile DOES have a legitimate argument that this messaging increase does NOT affect customers with the Unlimited Messaging Add-On (since no measurable effects will be felt by the customer on their future bills). Therefore using "increasing messaging charges" as an excuse to waive the ETF on accounts with an Unlimited Messaging Add-On is a BIG YMMV!
Also, when T-Mobile does implement controls that allow customers to block unwanted messages (which according to rumor should be in the near future), using "increasing messaging charges" as an excuse to waive the ETF on ANY T-Mobile account (regardless of messaging add-on) will be INVALID. This is because messaging charges will NOT BE MANDATORY anymore, since there would be a user-controlled way to opt-out of the network's messaging feature.
2) Customers using this "increasing messaging charges" excuse WILL NOT BE ABLE TO STAY MONTH-TO-MONTH! Your options are: A) cancel your line and lose your number, or B) port your number to a different carrier.
3) If you are using this excuse to leave T-Mobile for another carrier or because you don't need the number/line anymore, this is a pretty good opportunity to do that. Better do it this time, as in the future "increasing messaging charges" may not be a valid excuse anymore (see #1).
4) If you are using this excuse to get a new phone at "new customer" prices either for personal use or profit AND you want to stay with T-Mobile, then this may NOT work for everyone due to the following reasons: A) T-Mobile will NOT let you stay month-to-month after using this excuse (either port out or cancel line), B) T-Mobile will only consider you a "new customer" if you DIDN'T have T-Mobile for the previous 90-days or more, and C) the LONGEST "trial period" any carrier gives in the United States is 30-days (correct me if I'm wrong).
Therefore, if you are using this excuse to get a new phone, this would only work IF you could: A) cancel your line and sign up using SOMEONE ELSE'S NAME (ie: new family plan in spouse's or relative's name), or B) if you could survive on a PREPAID phone for 90-days (but if you don't use their phone that much in the first place, then you probably should be on a prepaid plan to begin with to save money), or C) if you switched to another carrier offering NO-CONTRACT options for 90-days or more (only a couple carriers offer no-contract options upon sign-up). For options B & C, after 90-days, you can sign back up with T-Mobile as a "new customer".
5) Usually T-Mobile will have you wait until the official announcement is out and printed inserts/statements are included with the paper/electronic bills BEFORE cancelling/porting your line without ETF. However, it MAY also depend on which operator you get and how nice you are to them. If you can't wait for the official announcement, you can try getting out of your contract ETF-free sooner, but YMMV.
6) And usually once T-Mobile makes up its mind to change the contract (ie: increassing messaging rates), they usually start notifying any new customers signing up about the change and effective date. Therefore, for anyone thinking about signing up with T-Mobile right now for a free/cheap phone and then cancelling their contract ETF-free using this excuse, I would say chances are extremely slim-to-none...
Whew... that took a while to write... but I hope this helps (at least someone)! 
EDIT: Added #6.