Scott Horton in The New Republic suggests there are "unsettling issues" about "whether a selective attitude is taken in prosecution--that is, whether the Justice Department is treating Spitzer in a manner consistent with other (notably Republican) figures caught in a similarly compromised position."
I don't know enough about the subject to comment on the big picture question, but I did want to point out one item. Horton writes,
PIN has emerged as one of the most highly politicized branches of a highly politicized Justice Department. According to a study done by two university professors, under President Bush PIN has initiated 5.6 cases involving Democrats for every one case involving a Republican. This statistical data strongly suggest that PIN has a habit of aggressively pushing cases on the basis of partisan political criteria.
But Horton doesn't point out that for the most analogous class of officeholder -- "State-Wide and Federal Elected Officials" -- the study reports (emphasis added) that "there is not a significant difference between the number of federal investigations of State-wide and federal elected officials and what would be expected given the representation of Democratic and Republican office-holders in the population." The breakdown of investigations there was 36 Democrats and 30 Republicans, which is almost identical to what would be expected given the ratio of Democrats to Republicans in those offices (33 to 27, according to the study).
I haven't examined the rest of the study carefully, and can't speak to the significance of its findings about local officials, where there is a large Democrat/Republican disparity; there may well be some bias there, though it's hard to tell without further investigation. But it seems noteworthy that this statistical study's empirical findings as to the officeholder category that's most analogous to Spitzer's do not support the inference for which Horton is using the study.