EDIT: As of 2010 SP3, the following is no longer a problem. Thanks for listening!
The mirror tool used to be awesome. You click the icon and it defaults to the Pick option. You changed your mind in the middle of the command? No problem…just click the pencil button on the Options Bar to draw your mirror axis.
In 2010, this beauty gets thrown out with the bath water. You change your mind in the middle of the command? Tough, start over…pick your elements from scratch and re-mirror with the correct option. Clearly a step backwards and frankly I’m tired of it! It can be easily corrected because a similar approach already exists when using the Dimension tool.
When mirroring, the Modify contextual tabs are obviously different than the Place Dimensions tab, and will further differ based on the combination of selected objects. Once you start the Mirror command, the panels grey out, but I suppose they could give us another panel with different mirror axis options or present us with the old buttons on the Options bar. Clearly, this is another UI problem that needs addressed.