I'll say that i thought it sounded familiar when i first heard it, but I couldn't place the other song so alas I decided that I was probably just remembering the snippets Mika gave us in his blogs.
Seeing this, however, I still don't believe there is plagiarism. It is a fairly common chord progression found in more songs other than WAG and heaven is a place on earth. I think WAG fun, upbeat and entertaining, while HIAPOE (ha, funny acronym:teehee:) is meloncholy, kind of happy but mellow.
Yay!
I feel the need to add also, that in reding the book "how to read literature like a professor" there is a chapter called "there is only one story" about how literature has been so looked over that there is no more room for complete originality - in essence, everything has been written. The same may go for music, a chord progression here or there may match one of "this" song while one there may match one of "that other song" and so on. You just have to enjoy the different ways they have been put together because every chord, every note as been used before. yay again!