If RIM can achieve this without stability and compatibility issues, it's a brilliant move. Suddenly, you have just attracted two huge markets (BlackBerry and Android) into at least looking at one device.
I can't say this would make many Android users want to buy a Playbook but it could be good for existing BB users. What would Google gain by allowing official access to the Market? Possibly work out a deal with RIM to become the default search engine on the Playbook. From a user standpoint this could be good because developers may be attracted to a multiplatform environment.