James C. Smith, of Reflexive Entertainment, posted some great data on the IndieGamer forums.
Click on the link for the full summary, but the nutshell is:
On Reflexive Arcade, the top 5 and top 20 games for the last 12 month period generate 19.6% and 42.6% respectively, of gross revenue. That’s a lot, but there’s still room to generate revenue outside of the top 20.
For a single month period, the numbers are a bit more skewed towards the top (since a handful of games are more likely to dominate a single month than a whole year), but still, not too bad. The top 20 games generated 54.0% of the revenue for September 2006.