With the official builds (as of r18415), you cannot charge the Sansas yet. If you plug one in, the battery percentage will shoot up to 100%, but it is not being charged. It is, however, running off the power source you just plugged in, so at least you're not draining your battery.
There
is a patch for charging the Sansas, but to use it you must set up a build environment any apply the patch to the source code yourself. With that said, I applied the patch about a week ago and have since been using Rockbox to charge, instead of the OF. It works very nicely.*
As saratoga already said,
We'll probably enable [charging] in a couple weeks when the feature freeze is over.
*Note: Currently the battery meter still shoots up, but not as quickly, and it stops at 99% for a while. Just let it sit there for a while (about two hours for a badly drained battery), even if it hits 100%, and it should be in the upper 90s after unplugging.