(In philosophy, solipsists do not believe anything external to their minds exist. In Egan-ese, the solipsists may very well believe in an external world. They just don't care much about it.)
The solipsists are typically software descendants of natural human minds. But in the "Wang Carpets" section of Diaspora, and the short story upon which it is based, Egan suggests the possibility of naturally occurring solipsists. In the story, enormous polysaccharide biocomputers (implementing Wang tiles) support the 16-dimensional virtual universe housing intelligent hyper-squids.
For a relatively concise description of Egan's "Wang's Carpets" see the opening sections of the Biocomputation chapter of Biomedia by Eugene Thacker. [Link to Google Books.]
Egan's story invites the question of whether such naturally occurring virtual minds are indeed possible. Besides just blind random luck, how would the requisite hardware substrate come into being? there may be a tendency to think of the virtual affairs thereby simulated as epiphenomenal, and thus inconsequential to any of the selection pressures that may otherwise result in the evolutionary emergence of the hardware substrate. But perhaps this assumption of the virtual as epiphenomenal is mistaken.