Alfie Dennen (of Moblog fame) has done a write up on Twitter's recent decision to stop the free SMS element of its service. I've seen quite a bit written about it and most of it lacks any understanding of how SMS or the operators work but this post makes some sense of it so worth a look.
For the record, I would pay to receive my (selected) DMs (that's direct messages to the twitter uninitiated) via SMS as well as email. I don't get a huge amount each week, but enough to warrant me buying some kind of bundle.
And for those on the facebook group on this topic who think that the pay to receive SMS on twitter is tantamount to paying to receive junk text messages then they clearly don't understand how the privacy and DM settings work - it doesn't *all* happen on SMS you know. Most of the interaction is web-based - be that mobile or full-fat internet.