The platypus (Ornithorhynchus anatinus) is one odd duck ...

... or maybe that's "one odd reptile", or "one odd marsupial"? Who the heck knows? Well, we all do now, thanks to the genome of the platypus being sequenced. First link is to a New Scientist article, the second is to a commentary by Nature magazine where the peer-reviewed manuscript is located.

Taken from the conclusion of the manuscript:

Since its initial description, the platypus has stood out as a species with a blend of reptilian and mammalian features, which is a characteristic that penetrates to the level of the genome sequence. The density and distribution of repetitive sequence, for example, reflects this fact. The high frequency of interspersed repeats in the platypus genome, although typical for mammalian genomes, is in contrast with the observed mean microsatellite coverage, which appears more reptilian. Additionally, the correlation of parent-of-origin-specific expression patterns in regions of reduced interspersed repeats in the platypus suggests that the evolution of imprinting in therians is linked to the accumulation of repetitive elements.

Translation: The platypus is obviously a mammal, but they are mammals which are much more primitive than just about every other mammalian species known to exist today, with the lone exception of the spiny anteater (echidna). Studying the genome of the platypus will give us important clues as to how the other mammals evolved.