An interesting synthesis of the Mars Express data:

Orbiter Prompts Revamped View of Mars

Some key quotes from that article:

"It's also uncovering mineralogical signs that are making it much harder to argue that the Red Planet ever had large lakes or seas, and certainly not within the last 3.5 billion years."

"It wasn't so warm and not so wet in much of its time."

"Among the evidence against Martian seas is the now-confirmed absence of any telltale spectral signatures of the water-formed minerals known as carbonates on Mars' surface."

"We have no convincing evidence yet of subsurface liquid water."

"In fact, said Bibring, it's the loss of that magnetic field that might have exposed Mars to more severe solar radiation and triggered the climate change 3.5 million years ago that left it seemingly barren today."