Astrobiology magazine has a short article about the hunt for planets outside the solar system. The most successful search method has found lots of Jupiter-plus planets, which are big enough to cause a noticeable wobble in the parent star's position. Refinements of this approach have led to detection last August of planets as small as Neptune, i.e. the theoretical smaller limit of an "ice giant".
However, all three of these "small" planets that have been successfully detected orbit very close to their parent star-- closer than Jupiter in our system. This may be an artifact of the search procedure (close in makes for more rapid stellar wobbles) but still, ice giants should be forming much farther out because it's too hot close in. Instead, they could be supersized rocky versions (similar to Earth's composition) or they could be icy despite their nearness, having migrated inward from a more distant birthplace. The astronomer Alan Boss is betting on a super-sized rock, especially because in 2 of the 3 systems where they've been seen the same system has a Jupiter-like planet further out. These big planets would be a big obstacle for an ice giant trying to migrate inward.