Saurahb and karlbecker_com's solutions are great, though they can cause an obvious popping effect when the view contains a tableview while the tab bar animates back up. I've made some modifications and combined it into a single function (as a category on UITabBarController). It's not completely perfect (delayed correction animation) but gives good results with tables.
If you like animation blocks and categories, give this a try. Orientation and device friendly.
UITabBarController+ShowHideBar.m:
#import "UITabBarController+ShowHideBar.h"
@implementation UITabBarController (ShowHideBar)
- (void) setHidden:(BOOL)hidden{
    CGRect screenRect = [[UIScreen mainScreen] bounds];
    float fHeight = screenRect.size.height;
    if(  UIDeviceOrientationIsLandscape([UIApplication sharedApplication].statusBarOrientation) ){
        fHeight = screenRect.size.width;
    }
    if(!hidden) fHeight -= self.tabBar.frame.size.height;
    [UIView animateWithDuration:0.25 animations:^{
        for(UIView *view in self.view.subviews){
            if([view isKindOfClass:[UITabBar class]]){
                [view setFrame:CGRectMake(view.frame.origin.x, fHeight, view.frame.size.width, view.frame.size.height)];
            }else{
                if(hidden) [view setFrame:CGRectMake(view.frame.origin.x, view.frame.origin.y, view.frame.size.width, fHeight)];
            }
        }
    }completion:^(BOOL finished){
        if(!hidden){
            [UIView animateWithDuration:0.25 animations:^{
                for(UIView *view in self.view.subviews)
                {
                    if(![view isKindOfClass:[UITabBar class]])
                        [view setFrame:CGRectMake(view.frame.origin.x, view.frame.origin.y, view.frame.size.width, fHeight)];
                }
            }];
        }
    }];
}
@end
UITabBarController+ShowHideBar.h:
#import <UIKit/UIKit.h>
@interface UITabBarController (ShowHideBar)
- (void) setHidden:(BOOL)hidden;
@end
Usage: 
[self.tabBarController setHidden:YES];
[self.tabBarController setHidden:NO];