I vacuumed the dust out of my SEL 18-200mm lens.

In the meantime, this kit, available from REI, has two rubber O-rings that will fit perfectly into the "gap" between any NEX lens and the lens mount:

http://www.rei.com/product/407235/msr-water-filter-maintenance-kit

I don't think you can find anything that will fit as well as the O-rings included in this kit will.

And, it's very obvious that dust gets into the lens via this gap.  When you try to vacuum dust out anywhere else around the lens, nothing happens.  It'll only come out if you vacuum from the bottom of the lens.  Thus, that's how the dust has to be getting in there.  And hopefully keeping something on there to close that gap will help.

Tom Hoots
http://www.pbase.com/thoots
http://www.dpreview.com/galleries/4330317199/albums