iTunes 10.5 is the latest iTune from Apple which support iOS 5. Currently it is beta stage and not fully released.
For those who want to know how to upgrade to iOS 5 see this link.
iTunes 10.5 Direct Download Links
For Mac Users : iTunes 10.5 for Mac OS
For 32 bit windows users : iTunes 10.5 for Windows 32 Bit
For 64 bit Windows user: iTunes 10.5 for Windows 64 Bit