Be vigilant about managing contracts and stating what you support. I once inherited a contract that had us supporting "Macintosh OS 8.1 and later." This committed our marketing team to a future that we didn't want to embrace. More drastically, it created a nearly impossible task for my development team! Practice vigilance when creating portable software with respect to contracts. At the very least, make certain that the versions and platforms you support are very clearly identified. You may also want to specify individual support policies for various platforms, such as limiting the amount of time in which you will provide support for an older platform.