The new web interface attempts to convert email addresses put in the To/Cc/Bcc boxes on the compose screen into "tokens". However there were a number of bugs with the current implementation listed on our wiki bug page:
I've now completely rewritten the address parser, and put it on our beta server for testing. The new parser fixes all of the above problems, should be more resilient to odd address formats, and now allows you to do things like paste a list of email address with only spaces between them (rather than having to put commas) and it should tokenise them correctly.
If you come across any new problems, please email me directly at email@example.com with details including an example of how to reproduce the problem and I'll look into it.
Update 16-Apr-2009: Since there were no reported problems, I've now rolled out the new tokeniser to the production servers.