I recently downloaded "My OldBoy Free" a gameboy color emulator for Android phones, and am really impressed with it. With only a D-pad and two buttons, its still easy enough to control with a touch screen, and the 2x speed feature lets you avoid the often-slow pace of these classic games when you wish to. Supposedly you can also use a wireless 'link cable', which sounds cool but I haven't tried it yet - might only be the paid version.
Pokemon Blue was a favorite game in grade school, and playing it again is a lot of fun.
Nintendo could double their revenue if they released an official emulator in the mobile app store. I hardly ever spend money for mobile games, but would happily drop $5 per title. The quality and depth of these old titles puts almost every modern mobile game to shame.