About a month ago, Twitter was forced to disable the ability for users to get updates on their mobile phones in Canada. According to Twitter, the Canadian carriers were effectively doubling their charges to Twitter every month. I don't blame Twitter for disabling this service due to the cost.
However, I think the move on the part of the Canadian wireless companies was a stupid and greedy one (do they make any type of moves?), especially in light of the recent news that text messages cost them pretty much nothing.
Anyone receiving an update from Twitter via text message on their phone (either an @ reply or direct message) would be much more likely to respond via their phone, being charged for an extra text message. The end result would probably be more text messages sent, and therefore more revenue for the phone companies.
It is unfortunate that something could not be worked out, but I look forward to the solution Twitter says it is working on.