Sept 26th 2012 update :for erasing iOS6 base maps look here: http://blog.sumbera.com/2012/09/26/how-to-erase-ios6-maps-in-mapkit/ 

I have searched this and couldn’t find any answer, so here is my own research so far on this subject. You can disable MapKit base layers using 3 approaches:

#1 remove subview from MKMapView (quite bad as you will loose all overlays too)

#2 use undocumented function (bad too as this will be rejected by Apple approval process):

// get MKMapTileView from view hierarchy
UIView * mkMapTileView = [((UIView*) [ ((UIView*)[self.subviews objectAtIndex:0]).
subviews objectAtIndex:0]).subviews objectAtIndex:0];
// call undocumented method
if ( [mkMapTileView respondsToSelector:@selector(setDrawingEnabled:)]){
[mkMapTileView performSelector:@selector(setDrawingEnabled:) withObject:(id) NO];

#3 use method swizzle

inspired by http://www.mikeash.com/pyblog/friday-qa-2010-01-29-method-replacement-for-fun-and-profit.html

http://atastypixel.com/blog/making-uitoolbar-and-uinavigationbars-background-totally-transparent/

this method is fine as it is official method and enables you to ‘subclass’ a class that you don’t have access in compile time.

#import <objc/runtime.h>

// original method declaration
static void (*_origDrawLayerInContext)(id, SEL, CALayer*, CGContextRef);
// set up subclass in runtime..in some entering method
UIView * mkMapTileView = [((UIView*) [ ((UIView*)[self.subviews objectAtIndex:0]). subviews objectAtIndex:0]).subviews objectAtIndex:0];

Method  origMethod = class_getInstanceMethod([mkMapTileView class], @selector(drawLayer:inContext:));

_origDrawLayerInContext = (void *)method_getImplementation(origMethod);

if(!class_addMethod([mkMapTileView class], @selector(drawLayer:inContext:), (IMP)OverrideDrawLayerInContext, method_getTypeEncoding(origMethod)))

method_setImplementation(origMethod, (IMP)OverrideDrawLayerInContext);

// implement our Override
static void OverrideDrawLayerInContext(UIView *self, SEL _cmd, CALayer *layer, CGContextRef context) {
// possibly call original method if you leave it empty base google maps will not be displayed. You can draw custom content here as well...
//  _origDrawLayerInContext(self, _cmd, layer, context);

}