This has been happening for a few snapshots: When picking out an area and starting to build, chunks will start disappearing, leaving holes in the world. More chunks are affected the longer one plays.
Environment
Win7 x64, Nvidia 430, driver 334.89
[20:10:18 INFO]: Minecraft Launcher 1.3.10 (through bootstrap 5) started on windows...
[20:10:18 INFO]: Refreshing local version list...
[20:10:18 INFO]: Current time is Feb 28, 2014 8:10:18 PM
[20:10:18 INFO]: System.getProperty('os.name') == 'Windows 7'
[20:10:18 INFO]: System.getProperty('os.version') == '6.1'
[20:10:18 INFO]: System.getProperty('os.arch') == 'x86'
[20:10:18 INFO]: System.getProperty('java.version') == '1.7.0_51'
[20:10:18 INFO]: System.getProperty('java.vendor') == 'Oracle Corporation'
[20:10:18 INFO]: System.getProperty('sun.arch.data.model') == '32'
[20:10:18 INFO]: Refreshing remote version list...
[20:10:19 INFO]: Refresh complete.
[20:10:19 INFO]: Loaded 2 profile(s); selected 'Latest Version'
[20:10:19 INFO]: Refreshing auth...
[20:10:19 INFO]: Logging in with access token
Attached are screenies of the missing chunks near my tower base... ie, they're in my 'active' area most of the time.