I just installed Ubuntu 11.04 and I was able to use gconf-editor to change the shortcut key.
Make sure you're not running gnome-do! (If it's set to use your key combo, the Dash won't be able to bind to those keys.)
Open a terminal and run
Navigate to this key:
Modify the show_launcher key to a single key value.
I set the value to
<Super>space and now Super+space opens the dash instead of Super. Note that if you use a key combo, it seems to take a minute before you can use that to invoke the Dash. You also need to complete the key combo quickly.
If you want a more user-friendly interface, you can use the command line:
gconftool --set /apps/compiz-1/plugins/unityshell/screen0/options/show_launcher --type=string "<Super>space"
Or use compizconfig-settings-manager
sudo apt-get install compizconfig-settings-manager
In the Desktop section, select Ubuntu Unity Plugin. Set Key to show the launcher to be your desired keys. Again, it takes a minute before you can invoke the Dash.