James Grimmelmann has an interesting mediation on advertising and blogs at The Laboratorium. I found it via Froomkin who has a somewhat different take on the issue. I find myself agreeing with Grimmelmann, with an added concern about transparency and subterfuge. Linking to certain blogs, or simply maintaining them in your blogroll, can advance agendas with which you strenuously disagree, even if you are generally on the same side politically, see e.g. this and this. Those that use disguised links rather than overt advertisements to avoid being challenged on their hyprocrisy are pretty unethical, in my view (e.g. see comments by me here). Lawyer bloggers based in NY may need to be especially careful.