I am experiencing the same issue. Have talked to ESRI Tech Support and they don�??t seem to have a solution for me. In version 9.3.1 there was a tool called �??Create Mobile Basemap�?? which had the functionality you are looking for, but this geoprocessing tool no longer exists.
For version 10 there only seems to be 4 options for creating a basemap for your mobile project, none of which allow you to use your own mxd document with only ArcInfo liscencing.
One clumsy workaround we have identified is to create your basemap mxd layer as an operational layer and add it to Mobile Project Centre, but then remove the option to edit or search this layer. This method isn�??t ideal, but seems to be the best solution we can come up with until ArcServer is purchased.