Take a look at the top 10-20 products at RPGNow for this month. It's very possible that sales of individual PDFs are dropping while overall sales continue to climb -- which would fit in with the number of new releases growing while the overall sales grow.
Looking at the year as a whole, Ronin Arts is doing well. I'm looking at individual item sales.
Also, I've noticed that stand-alone RPGs are doing better at RPGNow than they have in the past.
EDIT: Out of curiosity I just checked Ronin Arts' 2004 sales and compared them to 2005. As I thought overall sales continue to grow -- by the end of August Ronin Arts' 2005 sales will surpass all of 2004's sales. So it's definitely per product sales that are decreasing and not overall sales.
Why do I think individual fantasy PDFs are seeing lower sales than other types of PDFs? There are just so many, many more fantasy PDFs to choose from.