Fun Size Bytes (from @Tj)

mail: in:inbox is:unread

echuckles:

thank you, gmail help, for answering my question as to what to type in my gmail search box to show only my unread mails.
NO thank you to YOU, gmail, for not making “show unread” a default tab in mail.

Enable “Quick Links” in Google Labs and you can make a link to in:inbox is:unread

OR

Use “Multiple Inboxes” (also in Google Labs) and use “in:inbox AND is:unread” as a parameter.