99% of income earners and up into the top.
CRA indicates that income splitting with a spousal loan is OK as long as the 'tranferor' of the invested funds is paid interest at the 'prescribed rate', and that the interest is paid no later than 30 days after the end of each calendar year in which it becomes payable. Details can be found in section 21 of the following CRA interpretation bulletin IT-511R. The deadline for 2012 interest loan is fast approaching, so act soon.
Check with CRA here for the current prescribed rate including the first quarter rate for 2013. Remember, even if rates go up (not likely for 2013) your prescribed rate is locked in over the life of the loan. An example loan agreement can be found at the lawdepot.com (fill out their online form).