From Space.com:
Poring over images on his home computer taken by the now-orbiting Mars Global Surveyor (MGS), Clarke said that there are signs of vegetation evident in the photos.
"I'm quite serious when I say have a really good look at these new Mars images," Clarke said. "Something is actually moving and changing with the seasons that suggests, at least, vegetation," he said.
Clarke is a brilliant guy, and I personally agree with the notion that science fiction and science fact are very often separated by little more than time. However, given what is known about the Martian climate and surface conditions, I am still having a hard time with the idea that there are current, active species of vegetation there.
Based on his description, it seems more likely that what he is seeing are changes in the surface coloration produced by drifting dust and sand, not vegetation. I have not yet been able to track down any of the images to which he refers, so I don't know, just a supposition.
Hey, some of the bugs growing in my nickel plating solution would grow just fine on Mars. Other earthling extremophiles would do well in volcanic vents, etc. on Mars.