You can read how to set defaults in XLSForm at http://xlsform.org/#default. For a geopoint you can set the default by including 4 space-separated values: latitude, longitude, altitude (0 works), accuracy (0 works). The user will be prompted to zoom to it on map launch. For geotrace or geoshape, include either a point as previously described or a list of points separated by semi-colons (;).
There are a series of changes to the geo question types planned and described in a Google doc linked from Collect geotrace and geoshape improvements. One of them is to automatically zoom in to the current location or the default location if it is set. Another is to adjust the default zoom level while collecting points. Please take a look and comment if there are any other improvements you'd like to suggest.