WordPress plugins that operate off of the standard WordPress plugin application programming interface (API) generally work as expected in WordPress MU because WordPress MU is built on the same codebase as WordPress. Therefore, on principle, plugins that work with WordPress also work with WordPress MU. The same cannot be said of WordPress MU plugins working on standard WordPress installs, however.
There are three main differences between WordPress MU and WordPress when it comes to plugin development: the concept of blog IDs, the use of different database schemas (which should not generally matter if you're using the plugin API), and the concept of multiple blogs as it pertains to plugins that use data from other blogs.
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