An interesting paper published online today in Geophysical Research Letters:

Lillis, R. J.; Manga, M.; Mitchell, D. L.; Lin, R. P.; Acuna, M. H.
Unusual magnetic signature of the Hadriaca Patera Volcano: Implications for early Mars
Geophys. Res. Lett., Vol. 33, No. 3, L03202
10.1029/2005GL024905
04 February 2006
Abstract

"It is thus likely that Mars had at least one of the following: highland volcanism in pre-Hellas times (older than any datable volcanic feature on the planet) or an active dynamo some time after the Hellas impact."

I thought they were absolutely certain that the Hellas impact occurred after the dynamo shut down, which is why the Hellas Basin floor wasn't remagnetized afterwards.

With the caveat that no one is "absolutely certain" about anything on Mars, the prevailing view, based on the MGS MAG/ER signatures over Hellas and the surrounding region, is that the core dynamo ceased before basin emplacement.

Having said that, however, in addition to Lillis et al. speculating about a "second dynamo," Schubert et al. [2000] posit something similar: a longer term dynamo (extending post-Hellas). Indeed, the latter state, "There is no unambiguous constraint on the dynamo turn-off time other than its absence at present."