You don't lose the game for having 0 or less life.
As long as you have 0 or less life, all damage is dealt to you as though its source had infect. (Damage is dealt to you in the form of poison counters.)
6/1/2011 Phyrexian Unlife won't affect damage that reduces your life total from a positive number to 0 or less. For example, if you're at 3 life and are dealt 5 damage, you'll end up at -2 life. The next time you're dealt damage, it will be dealt as though its source had infect.
6/1/2011 If you're at 0 or less life and Phyrexian Unlife leaves the battlefield, you'll lose the game (unless some other effect is keeping you from losing).
6/1/2011 If you're at 0 or less life, you can't pay any amount of life except 0.
6/1/2011 You can still lose the game for other reasons, including having ten or more poison counters or drawing a card from a library with no cards in it.