We are designing a cross platform open-source note application for Linux using wxWidgets in C++ or in Python.

But we have a few questions I need to try to find answers for.

1. should we use a RTF format for the notes, or should we try to use the open source format .odt? Our most important concern is that text formatting is preserved when users cut and paste from OpenOffice, Word, or from Firefox. Any recommendations on how to go about doing this?

2. We want to include a 'Post-It' note feature similar to that used in WORD (in Word they call it "add comment") or in Adobe Acrobat (where it is like a little yellow sticky note) You can click on the document and add a 'comment'. A yellow little box will pop up in which you can type the comment.
Any suggestions on how to make this?

We are looking for volunteer programmers familiar with wxWidgets, python and C++, database, gui implementation

If you'd like to contribute to our efforts, contact us!

I notice that your top concern is maintaining formatting between apps when cut and paste is used. From a UI perspective I would say treat that with great care. Formatting of text is important, for example it can highlight certain elements within the text, however it can also distract from the main body when over used. I would say focus on what you want to achieve, maybe that is an easy to use application that provides clear to read notes. In which case you may want to limit the formatting that gets brought across. For example you may decide that all text will be presented at the same font size and font family, so if anything is pasted across from another application the font may change to the one of the notes but any colour differences and emphasis changes will be retained.

This is just my thoughts but if that is the way that you go then you want to consider what you will allow and then what file format you will use. If you want to support pop-up elements (post-its) within the text then you will need a mechanism to support that.

Is there a need for your file to be interoperable with other applications? If so you may want to consider an open format like open office, however that may be overkill in which case you may decide to create and publish your own format. If you create your own I woudl suggest that xml should be your starting point.

Thanks for your suggestions.
I appreciate comments, ideas and suggestions from others who have experience in this.

The application must preserve formatting as much as possible. Many users have carefully formatted documents, or highlighted important sections of their documents, and need the formatting to be preserved if they are to adopt the use of a new application.

We will start with an XML format. But what else should we be aware of?

quoting your title it says CrossPlatform, but python/wxwidgets is hardly cross platform!