I know that pedestrian routing with garmin is not high priority, but just in case.. Visiting Bratislava I noticed some wierd route suggestions walking trough old town. The main reason seems to be that there are plazas, which are not routed trough as it's area without any road continuing trough. Have a look at http://www.openstreetmap.org/?lat=48.14347&lon=17.10748&zoom=17&layers=B000F... and for example "Hlavne Nametie" (the plaza at the middle). No pedestrian routing trough that as on garmin there is no road.
In OSM data, it's tagged as area=yes highway=pedestrian restriction=foot (Bad tag?)
Perhaps a change where the area is still drawn as area, but the roads would be drawn too?
I don't know internals, and obviously this object is handled as a polygon. But with the tag highway=pedestrian I would expect a routable way. Maybe mkgmap can be changed to handle this case as a polygon *and* a route along the edges of the polygon? So each connected way could be routed through the plaza.