I'm not keeping up--and that's a good thing.
It's an indicator of a growing market, and that is a good thing, almost by definition.
As someone who's getting behind, I'm about to mention things you may already know about. You may well think to yourself "Goodness, Melissa, it's about time, get with the program!" but, just in case, I will make a point of mentioning them.
First, there's a nice interview over at Tekila Vella's blog Tekila Unplugged, "Today's SL Breasts". I can't say I agree with Ms. Vella on some things, notably her singling out Mused, Lolas, and Ecorp as today's "main contenders" in the prim breast world, but then I'm no maven, just a prim breast user. The only ones who might know are the prim breast makers themselves, Linden Lab if they had the urge to monitor what prim breasts people have attached at any given time--and I'm guessing they have other priorities :)--and, perhaps less accurately, clothiers who cater to prim breasts.
All that said, the interview, with Tayla Arado, lets Ms. Arado give her opinions of the aforementioned triumvirate (triumfeminarate?), what she'd like to see in prim breast-friendly fashions, and why women should take consider prim breasts.
Speaking of Tayla Arado, Ms. Vella kindly links to Ms. Arado's blog, Modern Lady: Fashion for Breasts. I must mention that much of the imagery on Modern Lady is very much NSFW. That said, it shows some fashions that I have not seen elsewhere, and any new (to me, if nobody else!) source of information is good. Check it out... somewhere safe.