That is a common place to have a leak - and mine has been replaced recently by my mechanic (the car was in for A/C work and I had them address this while in the shop).

Also, make sure the oil drain tube that runs up to the bottom of the air filter assembly is seated correctly. There should be a little o-ring on the bottom of the air-filter assembly (not sure how well it works), but it's easy to knock to one side when putting on the air filter assembly - say after replacing the mount or a bad insulator...

if it's not aligned and connected, oil will run down the tube and make a mess.....

__________________
~shell
As of 2/2010:
2001 CLK55 0o\=*=/o0
13.6 @ 106mph
10K mi
1984 300SD
260K mi and going and going...
97 S600
46K miles
1991 Sentra SE-R (extremely dorked with)
www.se-r.net