Google has announced improved comment capabilities in Spreadsheets, similar to those available in Google Docs and Presentations.
The basic comment feature, which will be familiar to Microsoft Excel users, already available in Google Spreadsheets has been updated to behave exactly like comments in a Google Doc. If a cell has a comment in it, you'll see an orange triangle in the upper right corner and when you hover over the cell you'll see the full discussion.
The total number of comments are also tallied up at the bottom of the screen on the sheet tab, and hovering over the comment icon shows all the comments on that sheet.
And just like you’re used to with comments elsewhere, you can +mention someone to automatically include them in a discussion and send them a notification via email – and they can even reply to the comment without leaving their inbox.
Any comments that were created in spreadsheets before today are still available and saved as “Notes”. These are shown in your spreadsheet using a black triangle in the corner of the cell to differentiate them from the new discussion-style comments. You can also create new notes from the “Insert” menu if you need to leave a quick annotation on a cell.
The new comments feature will be rolling out to Google Apps accounts soon.