At last! It's here. Now we can select contacts or groups easier while composing. I was tidying up my contacts last week, so this will work great with my new organization! (that's what I hope at least)
http://gmailblog.blogspot.com/2009/08/composing-message-try-contact-chooser.html
I don't know why they didn't implement this feature before, since Hotmail and other popular webmail clients have it since decades ago!