Pocketnow which stated that Ballmer's use of the term "modest" was in reference to the supply and distribution of the Microsoft Surface tablet which is available only at Microsoft Stores and online. Microsoft says that Ballmer feels that the reception to the tablet has been "fantastic" which is why it soon will be found in more stores and in more countries. Considering the dollar amount that Microsoft is spending on promoting the tablet, you would expect it to be made more readily available through more retail locations.
In retrospect, it does seem to make more sense to use the m word in relation to the rollout of the tablet itself, especially in light of the absence of any actual sales figures. So we will go with Microsoft's explanation for now and wait for Microsoft's Q4 earnings report to see how well the Microsoft Surface RT has been selling.
source: Pocketnow, MicrosoftNews via WMPoweruser