i do think that nintendo's handheld market is very vunerable to the mobile (apple/android) market but that might be because i've become so enamoured with my iphone. i just can't justify a dedicated uni-tasker any more.
one of the most dangerous things about the mobile market imo, the hand held market last a good 5-6 years before a hardware refresh. the moble market refreshes every 2 years meaning it can more 2-3x faster. it's going to be hard for a dedicated handheld market to really keep up with the mobile market.