I do not officially support the use of Mono with Twitterizer, just because I almost never used Mono, and therefore I could not count on support.
There have been several Mono projects for the successful implementation of Twitterizer, most notably Smuxi (http://www.smuxi.org/main/). Although these are a few changes, their source code will be a good start if you want to change Twitterizer for use with Mono.
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