Once people start getting hold of the DS develpoment package(s?), will it be possible for them to port n64 games to DS format? Then when DS Flash carts are released, we could play n64 games?
Not that easy.
[1]Consider the DS' ARM9 + ARM7 CPU combo against the N64's R4300 + floating point unit + SGI vector processor and you'll find the the DS does not come to close to the processing power of the N64.
[2]The resolution of the DS LCD (256x192) is not even close to what a regular TV set can display (720x480 for NTSC and 720x576 for PAL). You could scale the picture but that would suck (text unreadable, blurry sprites....)
[3]The DS does not have the same controls as the N64. It would make all game depending on the analog stick pretty much impossible. Though you could have an on-screen N64 controller and move the stylus over the on-screen analog stick to control

[4]Finally some speculation: Nintendo will port most good N64 games to DS anyway in the same way they ported most good SuperFamicom games to the GBA.