Wiktionary defines both to-do and todo:
to-do (plural to-dos)
2. A task that has been noted as one that must be completed, especially on a list.
My to-do list has been growing longer every day.
todo (plural todos)
(US) A task yet to be done; an item on a to-do list.
You can use whichever you want, but be consistent. To-do is a little clearer, but hyphens are naturally lost as languages evolve (to-day, wire-less), so todo should be fine too.
Edit: Waggers makes a very good point that todo isn't yet as widely adopted as to-do. So you may be better off sticking to to-do.
Ngram of "todo list" vs "to do list" (+ "list" because to avoid the fuss and commotion meanings, and "to do" will include "to-do"):
Comparison of term on newspaper websites, but note "to-do list" also returns hits for "to do list":
Phrase guardian.co.uk bbc.co.uk nytimes.com cnn.com
todo list 16 3,370 27 174
to-do list 2,350 79,000 130,000 30,100