You read the above title right. To get unlimited text on your T-Mobile-To-Go compatible phone (unlocked 1900-band-capable or T-Mobile) follow the below steps. Note that you'll also get minutes at a flat rate of 15 cents. Unfortunately you'll lose T-Zones access (news and such) but you can still check your balance by dialing *999 and hitting SEND (I'm pretty sure that's the number :) ).
1. Get a T-Mobile To Go sim card from a place like boardwalkcommunications.com for about $14.
2. Run it down to below $10. This may or may not be necessary as one person did this at $10.80 so you can try step 3 before this but don't count on the transition working unless the balance is low enough. You can do this by...surprise...texting!
3. call T-Mobile Customer Care and have them convert your sim to the Sidekick To Go plan.
You can now send as many SMSes as you like and not be charged anything. Plus you have voice at a fixed 15 cents per minute. The reason this works is that the Sidekick To Go plan has this as well, except it charges $1 a day. What you get for that $1 is unlimited MMS and web access on your Sidekick, which is also pretty expensive. Anyway the reason you get text for free and no MMS or web is they won't\can't bill you for the Sidekick $1 a day part of the plan if the sim card is not in your Sidekick. The flip side of this, the side that doesn't allow web access, is that the Sidekick uses a different way of sending and receiving data than normal that, for web and MMS, aren't compatible with normal phones.
So you can't use this on your Sidekick if you have one. But you can use it on such text-friendly phones\devices as the ones below:
Motorola v100 (old but good for text, but don't do voice on this one, about $25 shipped)
Nokia 3300 (it's a music phone too with a color screen, about $55 shipped)
Have fun with this, especially for AIM over SMS (which works with T-Mobile To Go, or at least this type) and 411sms (I think that Google SMS doesn't work but no huge deal there as 411sms is about as good, just with a 10-digit number)