[Moto X Q&A] KidJoe:
Pretty much, so long as Straight Talk uses ATT or T-Mobile towers.

To make things clearer, let me quote something I posted in another thread..see -> THIS POST

Sorry, I didn't mention that ATT's HSPA also uses 2100 Mhz, but the VZW X also supports that, so you can get ATT HSPA on the VZW X.

Two other important things to keep in mind... LTE, and APNs.

The LTE bands used/supported by the wireless carriers differ. So while the VZW X supports LTE, it only supports Band 4 and 13, if the carrier you intend to use doesn't use one of those bands where you go, you wont get LTE.

When it comes to APNs, or Access Point Names, these are critical for proper data settings, MMS usage, texting, etc. Set incorrectly and your phone wont work right. In another thread, I explained how when I first got my VZW Moto X Developer Edition, and it had 4.2.2, putting my T-Mobile USA SIM in, resulted in the T-Mo APN being received automatically, and everything working. However, with 4.4 on my X, the APN isn't properly received and I have to manually enter, and MMS doesn't work. See my posts in -> http://xdaforums.com/showthread.php?t=2594302

In addition to the GSM bands the VZW X supports, the T-Mobile X, (as well as the GSM Developer Edition as they are the same phone), supports HSPA on AWS (1700/2100) which the VZW X doesn't have. Why is this important? When T-Mo started their HSPA/HSPA+ 3G/4G built out, they used AWS 1700/2100 spectrum for it. In 2013, T-Mobile started their "Network Modernization" plan underwich they are coverting (or refarming) their 1900 Mhz from usage for EDGE to HSPA/HSPA+. So as mentioned above, where T-Mobile still has their HSPA/HSPA+ on the AWS (1700/2100) bands the T-Mo X will get and fully utilize HSPA/HSPA+, and the Verizon X will fall back to Edge. However, in areas they have refarmed their 1900mhz spectrum from EDGE to HSPA (aka "network modernization"), you'll get HSPA/HSPA+ with the Verizon X.

The T-Mo X also supports LTE bands 2 and 17, in addition to Band 4 that the verizon X also supports. So as you can imagine, if T-Mobile is using Band 2 or 17 for LTE where you are, the VZW X wont be able to use it. If TMobile is using band 4 for LTE in your area, the VZW X will be fine.

When it comes to the ATT GSM vs T-Mobile GSM band differences... It comes down to... Like the VZW X, the ATT X lacks HSPA/HSPA+ on AWS. And the LTE bands supported are slightly different. It was posted in another thread that the T-mobile (and thus GSM Developer Edition) X is missing two LTE bands when compared to the ATT X, however it was said that the two missing bands are not used in the USA.

---------- Post added at 08:01 AM ---------- Previous post was at 07:59 AM ----------

From the posts I've seen around the net. No one except Sprint can SIM Unlock the Sprint Moto X. To do so, your X must be currently active on Sprint.

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!UPDATE! 1/31/2014...

I do need to add something based experiences yesterday when I put my T-Mobile SIM in my Verizon Moto X developer edition (Kit Kat rom) again.

With the T-Mobile SIM inserted, and the phone reporting LTE, trying to make a phone call fails, service is briefly lost (4GLTE/3G indicator next to the bars goes out), Forcing the phone to LTE/GSM/UMTS doesn't help. Using an app like LTE OnOFF to go into the service menu and force the phone to either LTE/GSM auto (PRL) or LTE Only does not help, and the phone reverts back to LTE/GSM/CDMA auto (PRL)

There is a bug posted in another thread about not being able to use simultaneous voice/data while LTE on T-Mobile. I do not know the details on it, but its possible this is exactly what I'm seeing. Its posted in another thread that I don't have the link to right this second. Thread link -> http://xdaforums.com/showthread.php?t=2578665 just start reading around post 13 and 14... The work around was to force the phone to HSPA.

It was suggested to me by CellZealot to change the build.prop line of LteOnCdmaDevice to LteOnGsmDevice I have NOT tried this yet. I haven't had time, and VZW is my primary carrier, so I'd need to make the edit to test, then undo it to switch back to "normal" use. There may be other settings in there which need edited to be used in "pure GSM mode"

Also, I found that ATT also might be using LTE Band 4 in some areas, but I do not have a map showing which band LTE is used where, so while the VZW X may get ATT LTE in some areas, it wont pick it up everywhere. Also, I'm not sure how much Band 4 LTE ATT might have left due to the spectrum swap ATT did with T-Mobile as a result of the failed take over attempt. I have not had a chance to try an ATT Sim in my VZW X.