I've never heard of this claim, but it honestly doesn't sound that similar. It's a fairly simple song and I bet that if you looked hard enough you'd find a dozen songs that sound like the Spirit song.
A big resounding meh.
I'm a huge Zeppelin fan. They're one of my favorites and there's no denying their greatness. However, I know they're more than a little guilty of lifting stuff here and there. Any blues fan can attest to that.
Spirit can go **** off. Sour grapes.
The claim regarding Stairway sounded a bit thin. I thought the case for Dazed and Confused was stronger. That said, who knows where the earlier Dazed and Confused came from?
It's not what you steal when it comes to influence, it's what you add and the LZ version of Dazed and Confused was a lot stronger.
I recall reading in a book about the great Tin Pan Alley song writing teams about a pair (don't remember the names) who had a weekly radio show that demanded one new song per week and they were asked, "Didn't that take a lot out of you?" and the reply was, "No. But it sure took a lot out of Bach, Beethoven and Mozart!"
This has been going on for as long as people have been writing songs and as long as it isn't too egregious, I don't see a problem - it's what you add, not what you take that matters.
On the same topic, when I introduced my daughter to Stairway, she said, "Oh! They've taken that bit from Maroon 5..."