Existing smartphones max out at 32GB with the SDHC standard, which is why a new standard was needed. Obviously, SDXC is backwards compatible, in that an SDXC slot will happily read cards formatted as SDHC or even standard SD, so your old cards won't be obsolete come 2012 or 2013.
From the SanDisk product page:
Get faster performance and maximum storage for your photos, apps and Full HD† videos. With 64 GB capacity and transfer speeds up to 30 MB/sec, SanDisk® Mobile Ultra™ microSDXC™ memory cards are perfect for the latest microSDXC™-enabled high-performance smartphones and superphones - plus the included SD™ adapter lets you quickly transfer files from your PC or other SD™ enabled device - so you can get more out of your mobile device. Do more with SanDisk® mobile memory cards
- Store more with 64 GB storage capacity
- Faster performance with transfer speeds up to 30 MB/sec
- Easily transfer files to and from your PC with the included SD™ adapter
- Organize and transfer your photos, videos and music with included SanDisk® Media Manager software