In market client on phones at least
featured apps with high ratings get to
display the promotional graphic.This is the one that shows up on top
even before you start searching the
market for a specific app.
See this answer from Android market forum.
Edited: One of the google employee gives some clarifications here
Update: Both links above are now broken but the detailed information can be found here
Selected applications have the ability
to be featured atop their respective
categories. This is not a guaranteed
feature, but uploading promotional
graphics is something that we
recommend.