Looks like SEGA have figured a way around that pesky Sonic & Knuckles lock-on technology problem, as Japan is about to get the classic mushroom-shaped Mega Drive game soon on the Wii Virtual Console, complete with backwards compatibility across other Sonic MD games.
SEGA Japan announced the lock-on feature workaround, which involves an additional menu before you boot the ROM up asking you if there is another purchased Sonic game you would like to connect it to. Nintendo itself made no mention of the feature, which led us all to worry that the VC version would have nixed lock-on play – as has been the trend these days with Sonic & Knuckles re-releases. Sonic & Knuckles on Wii Virtual Console will hit Japan on the 27th October for 600 Nintendo Points.
The original Mega Drive classic was released in October 1994, eight months after the launch of Sonic the Hedgehog 3. As the two games were confirmed to be two parts of the same game, Sonic Team created a special cartridge that allowed copies of Sonic 3 to be ‘locked on’ the top of it. As a bonus, copies of Sonic the Hedgehog and Sonic the Hedgehog 2 also worked on the lock-on; the former as an ‘endless’ gauntlet of special stage game ‘Blue Sphere’ and the latter allowing players to control Knuckles in Sonic 2 Zones.
Source: Andriasang (thanks for the tip, BassDS!)