Samui? Sabui?
Samui desu ne.
Sou desu ne.
During my first winter (September - May) in Hokkaido, where the temperature occasionally drops below -30 degrees (Celsius), I had just gotten used to this everyday greeting, when it seemed that all of the sudden everyone was saying "sabui" instead of "samui."
When I asked my coworker, she explained that "sabui" is often used during periods of extreme cold. The reasoning is that when one's nose is congested, or frozen shut entirely, the Japanese semi-nasal "mu" will slide towards "bu". Hypothermia will probably accellerate this process, as well. This mispronunciation has become so widespread that it's become acceptable.
While the above pseudoetymology seemed plausible, sabui seems be nothing more than an older variation of the same word. Koujien lists an example sentence from Ukyo-buro, a book written in the 1800s.
At any rate, they can both be used, although sabui sounds a bit more oyaji-kusai.
Other meanings and usage
As my esteemed colleage notes above, samui can be used in response to a joke, and is often complete with a shivering guesture. In this case, samui refers to humour that is old, cliched, poorly delivered, or just generally dasai (lame).
This usage probably stems from a slightly archaic usage of samui to mean "barren," "poor" or "insufficient". This usage is still seen in some idiomatic expressions, such as futokoro ga samui, "I'm low on cash," or literally, "(my) breast pocket is cold."
As in the above idiom, samui is generally used to refer to a space, rather than an object. For example, a room, the weather, or your bed would be samui, while a drink, some soba, or your feet would be tsumetai. Also note that samui is never used to refer to someone or their personality, as in the English "cold-hearted", "cold-blooded", "a cold look". Once again, tsumetai is often used for this purpose.
Miscellany
The kanji for this word, 寒, pronounced samu-i (kun-yomi) / kan (on-yomi) is a grade 3 kanji and very commonly used in written Japanese. It is also quite common in place names within Hokkaido, such as Lake Akan (阿寒湖), Tsukisamu (月寒), Wassamu (和寒) and Hassamu (発寒). These names were adopted from the Ainu pronunciation, with the kanji for "cold" being a natural choice given the climate.
Samui is a winter kigo in haiku.