Okay so if you go to the Telenav OTA site at ota.telenav.com/tn and click T-Mobile as your carrier, then click the Vibrant 4G, it is completely compatible with the Captivate. Basic navigation is now free (including voice navigation, automatic re-routing, etc) and the full premium navigation (which includes SPEED TRAPS among traffic and all sorts of other things) is only $2.99 a month - MUCH better than paying at&t $9.99 a month and this app is much better than the stock at&t Navigator app and worlds better than Google Navigation, which quite often thinks you aren't on the right road when you're dealing with a highly populated area with lots of roads.
Also, unlike Google Nav, TeleNav will continue to work without a tower signal, it will just tell you to turn without the street name. I know this from experience when I drove from Colorado to Arizona and was driving through Hatch, NM and had no signal from Sprint or any roaming partner and still got navigation directions.
Just thought I'd post this up here. I decided to check out the OTA site and see what's up.
Also, if you download it directly from the device, it will tell you it can't recognize your device and you just hit Samsung, then Vibrant 4G.
I do not know if this works on non T-Mobile ROMs but I don't see why it wouldn't. Let me know what you guys think.