Can you really? I thought the "140 character" restriction was due to the "160 bytes in an SMS" restriction, and therefore using multibyte unicode characters consumed multiple "characters."
In practice, tweets get fragmented into multiple SMS anyway, so while this might have been the aim, it's not the implementation they've ended up with.
Also, while you may wish SMS was Unicode, that's not actually likely.
The number of characters you can fit in one SMS will vary depending on whether you use more than one charset, and what your operator and device support. Most SMS in the US/UK/Aus etc will be sent as 7 bit GSM, allowing for 160 characters. 8 bit allows for 140 and UCS-2 allows for 70. (This is assuming no UDH, i.e. no fragmentation)
(Yes, you can bankrupt someone by sending them a bunch of MMS)
It's also why Twitter is really popular in Japan. 140 characters is more than enough to hold actual conversations (even if most statements/responses are typically 12-20 characters and the rest of the space is used for emoji)
http://latimesblogs.latimes.com/technology/2009/05/invented-...
It's rather interesting that the 160 character limit isn't arbitrary - but also doesn't account for eg: URLs in text.
I don't know why that is, because I'm pretty sure you're right that the tweet length limit comes from SMS limits. But these days the 140 limit seems to be in terms of Unicode code points, not bytes.