Rotordynamic analysis can be substantially affected by aerodynamic loading effects which have been historically difficult to model or predict analytically. For actively controlled magnetic bearings, such model uncertainty (structural and parametric) is of great importance and the successful control scheme must exhibit sufficient robustness in the face of potentially destabilizing effects. Here, we model a hypothetical single stage centrifugal gas compressor with a single annular gas seal supported on two active, radial magnetic bearings, and control the rotor with a robust nonlinear compensator. A full-order, multivariable gain-scheduled observer designed via eigenstructure assignment techniques provides effective state estimation for use in the control law. Although the system described is admittedly simple, it has been conceived both to exhibit the necessmy behavior, and to foster discussion on the practical aspects of controller robustness to unmodelled effects.