Update: According to Windows Phone App List and Marketplace Browser the 10,000 app milestone has been passed. As reported below, this was achieved much faster than Android and on pace with the App Store.
WindowsPhoneapplists.com reports nearly 1200 applications have been added in the last 2 weeks, which is an acceleration on the usual 1000.
While the quality of apps is as important as the number, the WP7 Marketplace has already surpassed lesser rivals like webOS some months ago, and if one ignores the 11,000 spam apps (ebooks and themes) in Blackberry App World is nipping at the heels of its much larger rival.
It took the Android Market 11 months to reach the 10,000 app mile stone, something the Windows Phone 7 Marketplace is set to achieve in little more than 4 1/2 months.
In fact the Windows Phone 7 marketplace will hit the milestone even faster than the iPhone app store, which did it in 142 days, according to 148apps.com. At the time Apple took out a full page newspaper ad, but of course goal posts have moved on a bit since then.
The question of course remains whether growth will go exponential or remain linear, but there is no arguing that this is a very good start.
Thanks Kevin D for the tip.