Another issue linked to the closed source nature of flash is that, unlike for example movie files, there is not a simple recoding solution to covert flash files designed to play on your PC's 21" monitor down to a resolution and frame rate that the DS could handle.
As of right now, the DS and DSi can't even do flash. But, well, flash is adobe. Silverlight is microsoft. Come on, Nintendo, make your own version for us to use! XD
It will never happen. Just look at the PSP you could whach youtube on it but every 30seconds it just freezes and you have to w8 another 20 seconds for it to buffer and then it plays for 5 seconds than freezes. If the PSP could barely do it how will the DS.