Decomposition products of magnesium ethanoate
EDIT: Ok, here's your answer (Thermochimica Acta, 78 (1984) 17-27): It decomposes into the oxide, carbon dioxide and propanone: 

$Mg(C_2H_3O_2)_{2(s)} \rightarrow MgO_{(s)} + CO_{2(g)} + CH_3COCH_{3(g)}$

PRE-EDIT:

The Russian Wikipedia favors the second reaction. This seems to support the idea. 

This site mentions magnesium carbonate and propane-2-one as products, but this seems to be questionable, given the other sources.