I think I may have found a very simple and elegant approach to working with character sheets that will appeal to a lot of users here, supporting formats including PDF, .PNG (.jpg, etc), and Word/Excel. It's a free document annotation and markup service, nicely presented in HTML5, called Crocodoc which can be accessed at <a href="http://personal.crocodoc.com" rel="nofollow">http://personal.crocodoc.com</a>. You can use it anonymously, or you can create a free account that gives you access to personal document management, ala Google Drive (whether relevant or not, they are apparently partnered with Dropbox).
I've only just started playing with it this evening, but it seems like *exactly* the kind of collaborative RPG aid I've been seeking for a long time. Basically, you just upload your character sheet (or other handout) document in PDF/whatever, and then you have a suite of tools at your disposal for adding editable text, drawings, highlights, strikeouts, and comments of arrow/box/text-selection types. Every edit you (or any other collaborator) make is logged in a history attached to the document in a flyout panel to the right. Once you've set up the document, you can click the "Share" button, which allows you to either copy a unique URL for the document, email the link to a collaborator, or embed the document using iframes in your game blog/wiki/etc.
As a quick test, I uploaded a blank official Pathfinder RPG character sheet in uneditable PDF format to Crocodoc, then simply copied the direct URL and then, inside roll20, attached it to a hyperlink (using the "open link in a new tab" option) within a character's roll20 sheet. Simply clicking the hyperlink opened up the editable character sheet in a new tab and allowed me to annotate and add in values at will. As the GM, I will be able to leave comments/notes/corrections on a player's character sheet in realtime and they will know said edits came from me. Any comments I leave also have a "reply to" option that allows the player to respond specifically to the comment. Finally, the Crocodoc character sheets can be saved out to a printable offline PDF file at any time. I'm very impressed so far, and I can see that this is likely to change the way I handle elements of my RPGs, online and off.
Hope this is useful!