Hi,
I'd really love to see how many mails are currently unread in my Inbox folder but I only get this information when I hover the mouse over the tray icon.

Did I miss an option in the settings? I'm running KDE 4.8.0.

Do you mean something like this? http://i.imgur.com/BPOyV.png

If so, all I had to do was make sure the unread message(s) were in my Inbox - and enable the KMail System Tray icon to Always be shown.

Exactly, this is what I'm looking for. I've checked the option that the icon visible all the time and sent me some mails from another account but the icon didn't change.

I've noticed this on my git master setup, sometimes the icon is not updated, often at bootup. I'm unsure of the cause.