The situation has changed since writing this answer: now that Firefox has added support in version 22, all major browsers now support accessing the clipboard data in a paste event. See Nico Burns's answer for an example.
In the past this was not generally possible in a cross-browser way. The ideal would be to be able to get the pasted content via the paste
event, which is possible in recent browsers but not in some older browsers (in particular, Firefox < 22).
When you need to support older browsers, what you can do is quite involved and a bit of a hack that will work in Firefox 2+, IE 5.5+ and WebKit browsers such as Safari or Chrome. Recent versions of both TinyMCE and CKEditor use this technique:
designMode
off and call focus()
on the textarea, thus moving the caret and effectively redirecting the pastedesignMode
back on, restores the user selection and pastes the text in.Note that this will only work for keyboard paste events and not pastes from the context or edit menus. By the time the paste event fires, it's too late to redirect the caret into the textarea (in some browsers, at least).
In the unlikely event that you need to support Firefox 2, note that you'll need to place the textarea in the parent document rather than the WYSIWYG editor iframe's document in that browser.