Looks like Juzam Djinn is back, baby!
Phyrexian Vatmother has the classic Suicide Black profile - big monster, big downside, ability to end the game quickly, and that 2BB casting cost that we are so endeared to. Phyrexian Vatmother is sure to have a big appeal with the casual players who drive the game, and I expect these will trade very well. That, coupled with the possibility of having a poison deck in Standard, makes the Vatmother a pretty hot card. The downside of poison counters from the Vatmother is negligible - although poison will kill you twice as fast as Juzam's pinging will, it can also scramble up the opponent twice as fast.
Mirrodin Besieged is the second set in the block, meaning that even if this doesn't deliver the goods for a tourney-caliber poison deck, we might get it in the third pack, so to speak. Quiet Speculation's Stephen Moss discussed the possibility of a poison deck in this Insider-exclusive, A Helping Hand. The Poison deck is perilously close to happening, and I suggest paying attention to cards like Phyrexian Vatmother and other helpers like Septic Rats; another spoiler here or there with the "infect" keyword could make poison-bearing monsters very popular.