well, you beat me to this as i was writing up a tutorial. i'll post it anyways. as david says the polar coordinates filter is key.
first you need to find or put together a large panorama. i searched on panorama and found this one.
notice that the grass on the left and right sides are at about the same level. the closer the better.
this is shown much smaller than the original which was 2875x600px.
first make sure there is an alpha channel by clicking: layer>transparency>add alpha channel.
now click: filters>distorts>polar coordinates.
then use the clone tool and smudging to blend where the two ends meet.
if you like select a rectangular area and paste as a new image.
you can find panoramas that are complete 360 degrees. the sides will match perfectly with these. they are a bit long but you can cut parts out of the center. you would need to find parts that will match when put together.
the above was done quickly. take your time and you will get nicer results.