I am waiting for them to come out with a ChromeBook with SD drives on the bottom of chrome book just for this. Thus one would mount a SD drive to run Apps on ChromeBook and have off line storage on the drive too.
The Samsung has already a SD-Slot in the front, but there you could only store Music and Videos for using it offline with the mediaplayer.
A Offline-App-"Cache" for Web-Applications isnt inplemented at this time
and some other Web-Apps (like TiddlyWiki) couldnt save, because they didnt got the right to access the SD-Card for saving.