IplImage * I = cvLoadImage( "myimage.tif" );
CvScalar pixel = cvGet2D( I, y, x );

cv::Mat I = cv::imread( "myimage.tif" );
if( I.depth() == CV_8U && I.channels() == 3 )
    cv::Vec3b pixel = I.at<cv::Vec3b>( x, y );
else if( I.depth() == CV_32F && I.channels() == 1 )
    float pixel = I.at<cv::float>( x, y );

Mat img;  // My input mat, initialized elsewhere

// Pretty fast operation, Will only create an iplHeader pointing to the data in the mat
// No data is copied and no memory is mallocated.
// The Header resides on the stack (note its type is "IplImage" not "IplImage*")
IplImage iplImg = (IplImage)img;

// Then you may use the old (slow converting) legacy-functions if you like
CvScalar s = cvGet2D( &iplImg, y, x );

cv::Mat mat = ...; // something
cv::Scalar value = cv::mean(mat(cv::Rect(x, y, 1, 1)));