Bloom, by Wil McCarthy. Somehow I missed this one when it first came out in 1998. Quite enjoyable with intriguing worldbuilding, though the climax runs by a little too quickly. Near-future setting, with humanity having precipitously fled from an Earth being remade into a constantly evolving ecosystem of fractal associations of Mycora (gray goo). Now, a single tiny ship needs to go back and see just what is going on ...