10 million songs in over four months doesn't sound particularly impressive to me, considering that 1 million were sold in the first week. Compared to the hundreds of millions (billions?) of songs that have been swapped for free in the same period, and the billion-dollar music industry, 2.5 million songs/dollars per month is a drop in the ocean.
I haven't used the service, but I'm hardly their target audience (I don't buy or download music). Unlike other music sales services, Apple has been rather forthcoming with their figures, and they aren't pretty.
In the first day April 28th iTunes sales were at 200,000 per day. By May 5th CNet were reporting that sales had topped 1,000,000 meaning 140,000 songs were been sold per day. By May 14th this figure had fallen to 125,000. While figures published in the The NY Times on May 28th translate the figure into 100,000 per day.
The decline continued from there. 5 million tracks had been sold by June 23rd meaning the average daily sales had now hit 89,000. The figure hit 6.5 million on July 22nd translating into 52,000 sales per day.
There have been about 45 days since July 22nd and 3.5 million songs sold -- about 80,000 per day -- so maybe things are picking up. A lot depends on profit margins, but it's hard to see iTunes as a real source of revenue for a $5 billion corporation until it starts bringing in 10 or 20 times as much money.