As described in this hint, you can use Spaces and Exposé together at the same time -- just press F8 followed by F9. But how can you bind both operations to a single key? Use a tiny AppleScript with GUI Scripting turned on (Enable Access for Assistive Devices in the Universal Access System Preferences panel), and Quicksilver triggers to assign it to a key. In my setup, I've assigned Spaces to F7, Exposé to F8, and Spaceposé to F11; it works like a charm. Here is the Spaceposé script in all of it's glory:

tell application "System Events"
  key code 98 -- F7 - Spaces
  key code 100 -- F8 - Expose All Windows
end tell

Here are the key codes for all the function keys:

• F1: 122
• F2: 120
• F3: 99
• F4: 118
• F5: 96
• F6: 97
• F7: 98
• F8: 100
• F9: 101
• F10: 109
• F11: 103

[robg adds: I haven't tested this one...]