'bißchen' used to be correct until 1996. Today the correct spelling is 'bisschen'. When a doubled consonant follows a vowel, the vowel is short ('i' is short in this case, 'ß' counts as a single consonant).

No, it's just the effect of different spelling rules. → http://en.wikipedia.org/wiki/German_orthography_reform_of_1996

Thank you kraut9 and Phil :)

Also, it is usual in Switzerland to use two "ss," even in published works.. too many official languages in one country ;)

Note that people that use lazy-deutsch usually do not use ß and always write ss. It's just not convenient to type on a non-German keyboard.