Using http://vocaro.com/trevor/blog/2009/10/12/resize-a-uiimage-the-right-way/ but with additions for iPhone 4 hires scale images. Also includes fix for 24bit non alpha images.

UIImage+Resize.m

// Returns a copy of the image that has been transformed using the given affine transform and scaled to the new size
// The new image's orientation will be UIImageOrientationUp, regardless of the current image's orientation
// If the new size is not integral, it will be rounded up
- (UIImage *)resizedImage:(CGSize)newSize
                transform:(CGAffineTransform)transform
           drawTransposed:(BOOL)transpose
     interpolationQuality:(CGInterpolationQuality)quality {
	
	// COTTON - adding in scaling for iPhone 4
	BOOL shouldScale = NO;
	CGFloat newScale = 1.0;
	
	if ([UIScreen instancesRespondToSelector:@selector(scale)]) {
		shouldScale = YES;
		newScale = [[UIScreen mainScreen] scale];
		newSize = CGSizeMake(newSize.width * newScale, newSize.height * newScale);
	}
	
	
    CGRect newRect = CGRectIntegral(CGRectMake(0, 0, newSize.width, newSize.height));
    CGRect transposedRect = CGRectMake(0, 0, newRect.size.height, newRect.size.width);
    CGImageRef imageRef = self.CGImage;
	
	// code to deal with Non Alpha images
	CGBitmapInfo bitmapInfo = CGImageGetBitmapInfo(imageRef);
	if (CGImageGetAlphaInfo(imageRef) == kCGImageAlphaNone) {
		bitmapInfo &= ~kCGBitmapAlphaInfoMask;
		bitmapInfo |= kCGImageAlphaNoneSkipLast;
	}
	
    // Build a context that's the same dimensions as the new size
    CGContextRef bitmap = CGBitmapContextCreate(NULL,
                                                newRect.size.width,
                                                newRect.size.height,
                                                CGImageGetBitsPerComponent(imageRef),
                                                0,
                                                CGImageGetColorSpace(imageRef),
                                                bitmapInfo);
    
    // Rotate and/or flip the image if required by its orientation
    CGContextConcatCTM(bitmap, transform);
    
    // Set the quality level to use when rescaling
    CGContextSetInterpolationQuality(bitmap, quality);
    
    // Draw into the context; this scales the image
    CGContextDrawImage(bitmap, transpose ? transposedRect : newRect, imageRef);
    
    // Get the resized image from the context and a UIImage
    CGImageRef newImageRef = CGBitmapContextCreateImage(bitmap);
    UIImage *newImage = nil;
	
	// COTTON - adding in scaling for iPhone 4
	if (shouldScale) {
		newImage = [UIImage imageWithCGImage:newImageRef scale:newScale orientation:UIImageOrientationUp];
	} else {
		newImage = [UIImage imageWithCGImage:newImageRef];
	}
	
    // Clean up
    CGContextRelease(bitmap);
    CGImageRelease(newImageRef);
    
    return newImage;
}