Can I access USB and Firewire drives connected to my Mac?

Yes.

Note, though, that if nobody is logged in on your Mac, external drives aren't available. If you log out for security reasons you might like to start locking your screen instead, or use OS X's Fast User Switching.

Alternatively, you can make your external drives available even when logged out by running the following command in Terminal:

sudo defaults write /Library/Preferences/SystemConfiguration/autodiskmount AutomountDisksWithoutUserLogin -bool true

Can I access shared files on other Macs, an Airport or my Time Capsule?

Yes, these show up in the Shared section of the Places panel/popup.

Can I access shared files on Windows and other non-Apple sources (ie. non-AFP)?

Yes, but only if your Mac is already connected to the shared disk.

• You can automatically connect to a shared disk by adding the disk to your login items (see System Preferences -> Accounts -> Login Items). This will make sure Presence can always see the disk if you leave yourself logged in.
• Otherwise, make sure you're connected to the required disks before you leave your Mac.