According to IGN Nintendo will start slowly rolling out demos for WiiWare titles starting this month. Nintendo CEO Satoru Iwata cited the WiiWare's chief problem--that customers only sign into the Wii Shop Channel when they know exactly what product they want.
Microsoft has enforced the demo program for Xbox Live Arcade, as every single title on the service has a trial mode that costs nothing (Wikipedia). While Sony doesn't share the same mandate, the company does have a large amount of PSN games demoed in their store.
We'll keep you updated as we hear more about this program from Nintendo of America and begin to see demos on the Wii Shop Channel. Hopefully, Nintendo will join the year 2006 and offer demos of retail releases soon, too!