“To redeem a Nintendo DSi Download Ticket number, such as for the Nintendo DSi Speak Channel, enter the number off the Nintendo DSi Download Ticket in ‘Settings and Features’ off of the main Nintendo DSi Shop page, then select Nintendo DSi Download Ticket.”
It seems Nintendo's tech support team is had a bit of a SNAFU, accidentally posting the above quote in the Support section. They've since removed it, but it is still cached by Bing (and most likely, Google). You can take a look for yourself here.
Although one must wonder why someone would need a download ticket for a DSi Speak Channel when the Mic is built in... I say everyone hold off on getting a DS headset until more details are revealed.