There's been some talk throughout the blogosphere about whether the distribution of promo CDs to bloggers is a good idea. I think both sides of the argument have been represented quite eloquently, but as an editor of a biweekly online zine, I still have mixed feelings. Ethically, I am against the idea of being a record company's "hired gun," a contracted publicist working for nothing but a free CD and the opportunity to attach a particular artist's name to your publication. And receiving promos generally puts some kind of pressure on the reviewer to write about those CDs and adhere to editor-imposed deadlines for the sake of timeliness -- even if the reviewer doesn't have much to say about the record (yet).
On the plus side, promos are indeed a nice little kickback -- you can sell the ones you don't want on Amazon (something you can't do with downloaded CDs) and consider that more of a proper "paycheck" than you might get otherwise -- and most web publications and review-oriented zines with a degree of integrity and individuality expect you to work for free anyway. Right now, my zine's not really big enough that it gets gigantic stacks of free CDs every day -- I have to ask for what I want, and I'd never request anything that I didn't think fit in with the zine's format. So there's no pressure to review the latest album by (god help us) Linkin Park. Instead, I scour through some of the release-date pages on the web and try to get a good sense of what's coming out and what's being reissued, or I'll listen to recommendations from publicists -- chances are, they'll have a small mp3 page for their client so you can immediately tell whether the artist appeals to you, and if not, hey, you're not under any weird verbal contract to cover that artist!
And no, you shouldn't feel any obligation to write positive things about promos. It might be a little mean and pointless to trash something by an unknown band (unless said band is truly fucking EVIL), but publicists will be gracious if you give their artists a fair and even-handed critique. And if the publicist turns out to be a hypersensitive asshole (I've had this happen too), then you don't want to promote his/her artists anyway! The only thing you lose is one measly industry contact. No big deal.
But as this relates to bloggers... I kinda agree with Tom Ewing that blogs are not traditional media outlets, and blog writing must remain as pure and personal as it possibly can. Promo CDs alone will not necessarily taint this "purity," but any blogger who takes part in the Tres Producers offer should be aware of the potential Faustian bargaining involved.