You can change the keyboard activation in the Spotlight preference (as the prior poster mentioned) or in the Keyboard & Mouse preference pane, in the "Keyboard Shortcuts" tab. The Spotlight pref pane only allows you to assign activation to F1-F12 and Cmd+Space or Cmd+Opt+Space. In the Keyboard & Mouse pref pane, you can change it to whatever you want. For example, it's been assigned Opt+Cmd+Space for the Spotlight Bar and Ctrl+Opt+Cmd+Space for the Spotlight Window on my Powerbook. LaunchBar gets Cmd+Space.

And yes, LaunchBar works under Tiger, and is still quite useful. Spotlight doesn't seem to learn your keywords, and as a result (on my computer) it always gives some graphic that contains the word "excel" priority over Microsoft Excel. With LaunchBar, I've trained it to launch just using the letter "X". Plus, Spotlight doesn't have in-bar folder navigation, custom queries, and lots of other features that makes LaunchBar so useful to power users. Spotlight is great for it's in-document search capabilities, but for launching apps LaunchBar is much, much better (IMHO).