MBProgressHUD
functions by displaying the hud as a modal overlay:
The MBProgressHUD window spans over the entire space given to it by the initWithFrame constructor and catches all user input on this region, thereby preventing the user operations on components below the view. The HUD itself is drawn centered as a rounded semi-transparent view witch resizes depending on the user specified content.
See: http://orfoundation.github.com/ORFoundation/Classes/MBProgressHUD.html
I've found ZAActivityBar
to be a good alternative to MBProgressHUD
as it doesn't intercept taps to the view. See the GitHub page for more information.