Based on the feedback from my previous post, I think I know what the issue may be between Twitter Tools 2.0 and WordPress 2.9.
I have a new version available that may address the issue that some people were having saving posts with Twitter Tools enabled in WordPress 2.9.
I believe the cause of this issue is as described in this ticket. Basically, a change in WordPress core in 2.9 was incompatible with some plugins that included the same JSON library.
I have not been able to fully test this and would like your help in doing so. If you have experienced these problems,
please download this testing version, and let me know your results in the comments. Thanks!
UPDATE: Get version 2.1.x which has the same fix.
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