You can control the types of content on your iPhone that are searched when using the Spotlight search on the iPhone that's available on the version 3.0 software. You can also control the order in which the search takes place.

To change the defaults, go to Settings, select General, then scroll down and select Home. At the bottom of this list that is shown, select Search Results.

This then shows a list of all the things that Spotlight will search on your iPhone. By default, everything is selected. The options are:

Contacts
Applications
Music
Podcasts
Video
Audiobooks
Notes
Mail
Calendar

You can tap any row to disable or enable searching search of that type.

Another thing to note is that the search takes place in the order of the contents that are shown. So in the list above, first it searches contacts, then applications, then music, etc.

You can tap and hold on the icon to the right of each row and then drag-and-drop the row to re-order the list and change the way the search happens. This way, you can put the slow or less important searches to the bottom so that they happen last.