I've seen the same thing when solomining w/ guiminer. It reports a share submitted for anything with difficulty 1 or higher, which would only work with a pool environment.

Try cgminer instead. It will show you the difficulty of any share it finds. If the reported difficulty is lower than 7.6 million (soon to be 8.6 million) you haven't found anything useful.

cgminer also reports the highest difficulty share it has found since you started the program, which illustrates how futile solomining with a GPU really is.

BTW, if you're just in it to learn, try solo-mining bytecoins instead. They're functionally identical to bitcoins but with a much lower difficulty.