If you're trying to get cell data from Excel (or any other table source) into a SharePoint list's Checkbox column in Datasheet view, the values in each source field must be delimited with the semicolon and hash characters (
;#). Each cell's data must also have a leading and trailing semicolon/hash pair. This last bit is counter any other delimiting style I've encountered, but a copy or cut-&-paste action will not succeed without the surrounding characters.
For example: If your source cell contains the values
c delimited by commas (
a,b,c), replace those values with
I've been able to quickly reformat large collections of values using an application's Find-and-Replace action, but I haven't yet found a way to get Excel (2007) to use Regular Expressions and wildcards without a lot of fiddling. I've gotten around this by copying the source values to a RexEx-capable text editor (Microsoft Visual Studio Express 2012 for Web, in my case). The
$ characters denote the beginning and end of a line, respectively.
Excluding the RegEx find-and-replace details, I found the answer to this question in a (now) four-year-old post to the microsoft.public.sharepoint.windowsservices Google Group. It is the only answer I've been able to find online, so I want to increase its visibility and findability by sharing it here. I know asking and answering your own question is a valid action on StackExchange sites, but this isn't my answer, so I'd like the community's input on the propriety of this post. If I receive feedback that this post is permissible, I'll remove all this long explanatory content :-)