It's a shame Mail can't display unread/flagged only messages .. or can it? Correct me if I'm wrong. Anyway, here are two scripts to filter your mailbox. Save these scripts inside Users -> your_username -> Library -> Scripts -> Mail Scripts -> anySubFolder. They'll appear under the script icon on Mail menubar. In case you don't know, you can also add shortcut keys to the scripts; I'm too lazy to tell you how, but you can open up Mail Help and find the instruction under Browse Mail Help -> Customizing Mail -> Using the Scripts Menu.
Read the rest of the hint for the scripts...
By the way, In Jaguar, the following "Show unread only" script works, but you can't add a shortcut key.
Show unread only:
try
tell application "Mail"
set theViewer to front message viewer
set theMsg to messages of theViewer
set msgList to {}
repeat with thisMsg in theMsg
if read status of thisMsg is false then
set the end of msgList to thisMsg
end if
end repeat
if msgList is {} then
display dialog "There's no unread message in this mailbox."
else
tell theViewer to set properties to {visible messages:msgList}
end if
end tell
on error the errMsg number the errNmb
if the errNmb is not -128 then
set the errTxt to "Error: " & the errNmb & ". " & the errMsg
display dialog the errTxt buttons {"Cancel"} default button 1
else
error number -128
end if
end try
Show flagged only:
try
tell application "Mail"
set theViewer to front message viewer
set theMsg to messages of theViewer
set msgList to {}
repeat with thisMsg in theMsg
if flagged status of thisMsg is true then
set the end of msgList to thisMsg
end if
end repeat
if msgList is {} then
display dialog "There's no unread message in this mailbox."
else
tell theViewer to set properties to {visible messages:msgList}
end if
end tell
on error the errMsg number the errNmb
if the errNmb is not -128 then
set the errTxt to "Error: " & the errNmb & ". " & the errMsg
display dialog the errTxt buttons {"Cancel"} default button 1
else
error number -128
end if
end try
Mac OS X Hints
http://hints.macworld.com/article.php?story=20031226013514930