I have this problem since 2 weeks too. Yesterday it went so much bad that the black screen didn't flashed back to normal - so at final step i just could restart my imac (ati card)!
SO WHAT IS GOING ON ? it went all time very fine - so nobody can say me, my mac is to bad! my second life vierw was perfect, until - this mesh stuff started to be official available!
so please, what can be done? =( it do bothering me and it make me disoppointed to get on this way pushed out of sl.