Quad band GSM GPRS and EDGE data will work fine. 3G and HSPA+ faux 4g will suffer while indoors. The device will still function, but the decrease in data speed will be noticable. I'm also expecting my AT&T Microcell Femocell device to save me while indoors at home. Supposedly having one of those is a viable way to get around the problem. Thing is you can't bring this device with you everywhere you go. Its only usable at the one location you configure it for. Internal GPS prevents its use in multiple or unauthorized locations.
Of course much of this information regarding the use of 3G / HSPA+ on 1900 MHz only in the USA is speculative or obtained indirectly from second hand sources. I won't be able to definitively answer this question until I receive and test out my EU Pre 3 here in the USA, Tuesday or Wednesday next week. Will post first hand results at that time.
Here is a coverage map of the 1900 mhz band for att the pre 3 will run