I've just started to look into DDM myself, and it appears that DDM may be best suited for short-term investments.

Here are a couple of links I found discussing the Ultra Shares strategy:

http://www.fool.com/investing/mutual-funds/2006/11/16/the-best-etf-for-2007-proshares-ultra-dow30.aspx

http://www.fool.com/investing/mutual-funds/2006/11/16/the-best-etf-for-2007-proshares-ultra-sampp-500.aspx

The prospecus for the ultra shares warn that over the long-term, it will be difficult for them to mirror 200% of the return for the underlying index, and that in flat or down markets, they will almost certainly underperform the index.

Best of luck to you, and let me know if you find out any more information.

Disclosure: Short-term long DDM