Why would Jesus be saying "I'm telling you today" as opposed to "I'm telling you tomorrow" or "I'm telling you yesterday"? Of course He was telling him that day, He was right there speaking to him. The argument about this verse that has cropped up doesn't make sense in itself. Nowhere else in Scripture does Jesus preface stuff with, "I tell you the truth today" but He says "I tell you the truth". I don't say to my son, "Josh, I'm telling you today that we're going to the store". I'd say, "Josh, we're going to the store today".
As for the assumption that Jesus did not go to Heaven that day because of the later verse "I've not yet ascended to my Father". Isn't it possible that He did go to Heaven after His death, but on the 3rd day when He was bodily resurrected He needed to return again to Heaven afterwards? After all, He came back and let people touch him later, so He's obviously going back and forth as He wills at that point. Why He needed to return to Heaven before anyone touched Him is curious; has anyone ventured a guess as to why?