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They really feel at home within this constraint. It's very important to them that it be compatible with the 160-char SMS limit because they feel Twitter is at its heart an SMS publishing system that has grown to have other uses.

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They may feel that, but they never really got that right in the first place. Twitter has a 140 character limit. SMS has a 140 byte limit. There's a 7-bit GSM character encoding which gives you the 160 characters, but that only covers basic English, select bits of other Western European langauges, and some currency symbols. So beyond those characters, you have to go down to 140 characters, and if you're in non-Latin land (like Chinese or Arabic), you have to use UCS-2, which only gives you 70 characters.

The simple Twitter 140 limit doesn't map to that at all, and coupled with the fact that Twitter SMS is disabled in a good chunk of countries, it seems silly to think of it as an SMS publishing system.

I believe the extra 20 characters is there to accommodate a username in addition to the tweet.

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