I just wanted to give a slight clarification for the other 5 star reviewers. I have a myTouch 4g using 2.3.4 so I did my homework before investing in this (and I've been waiting a long time for it to come out). I only purchased it after several other tech reviewers on other websites showed videos of their Android smartphones running 2.3.4 and after reformatting it inside the phone, working perfectly. After one tense minute of the reformatting process I was pleased to see a useable 59.3 gig of space available to use. Although you will not be able to take advantage of the SD XC format's claimed 30mb/s read/write speed after formatting it with your phone (google "will android smartphones be able to use 64 gig micro sd xc") it is a small price to pay for twice the available storage. That and you have bragging rights for the foreseeable future until the 128 gig cards come out for $500 or so.
One last note- to anyone who still doubts that it might work in their smartphones, the adapter that comes with it says "micro SDHC" not "micro SDXC" leading me to believe there probably isn't a whole lot of difference between the two if SanDisk is too lazy to make new adapters to make us think there is.