Wanted to add something to this and clarify/correct a few points:
Plus, while I'm sure someone in a lab has done it, you really don't run DWDM over multimode fiber - I'd second the opinion of it's cheap enough, go for the single mode and get the most flexibility in your options possible.
In fact, already being done - this is how 10GB-LX4 operates. The point here is that each of the four channels operates at less than 10 gigabits/sec, and that MMF didn't prevent it, in fact, it was done entirely to make 10 gig work over MMF. Caveats include mode-adapter cables and other funk to interface LX4 to mmf. Long-reach single-carrier (ie. single optical channel/frequency) 10 gig over MMF salso has a 'spec' (10G-LRM), but I'm not personally familiar enough with vendors to offer anything useful or practical.
One minor consideration is usually SM optics are stronger, so don't forget attenuation if it's a short distance or you might burn out your pricey new optics!
I would invite folks to examine the various gradations of gig and 10 gig LR/ER/ZR devices. Pulled from a handy table at http://www.andovercg.com/datasheets/juniper-ethernet-pics.pdf, I submit for your consideration a summary of the powers across the various flavors of xenpak. Note the modest increases in launch power, while there are considerable and huge increases in sensitivity. 10-Gbps Gigabit Ethernet XENPAK, 1-port • XENPAK pluggable optics (SR, LR, ER, ZR types) • SR optical interface (IEEE 802.3ae compliant) – Average launch power: -4.5 through -1 dBm – Receiver saturation: -1.0 dBm – Receiver sensitivity: -7.5 dBm • LR optical interface (IEEE 802.3ae compliant) – Average launch power: -4 through 0.5 dBm – Receiver saturation: 0.5 dBm – Receiver sensitivity: -10.3 dBm • ER optical interface (IEEE 802.3ae compliant) – Average launch power: -4.7 through 4 dBm – Receiver saturation: 1 dBm – Receiver sensitivity: -11.3 dBm • ZR optical interface (IEEE 802.3ae compliant) – Average launch power: 0 through 4 dBm – Receiver saturation: -7 dBm – Receiver sensitivity: -24 dBm -tk