As a gnome user I've become addicted to gnome do, and I think that a similar program would be awesome for the N900. Ctrl-space and win!
Anyway I looked into it a little bit and even though there is some old port of mono for maemo I concluded that it's not the way to go.
I found another launcher called Kupfer http://kaizer.se/wiki/kupfer/ that does not require mono and is pretty similar. After some tinkering and compiling of libraries (namely python-gobject_2.16 to get gio) I actually got it compiled on the sdk. It ran, but didn't actually work.
The usage of this application is very simple.
Press your keyboard shortcut (Ctrl-space for example) and a dialogue pops up. By typing it suggests an application to launch based on what you've typed, and enter launches that application. The list is prioritized in such a way that the most popular hits come up first. You should be able to scroll down the list of matches.
Some of these programs can perform all sorts of tasks, but the most basic is to very quickly launch an installed application.