*** SPOILER ALERT ***
ah, thanks for bringing that up anitram, i think the baby has to be Leo's -- because i don't think Matt was infertile, he was just having ED issues during this period in time (which might be why she hopped into bed with Leo so quickly), so that makes more sense that he's the father, though you'd wonder if Matt wouldn't have gotten suspicious.
not positive about the envelope -- perhaps information that could have proved that Leo was who he said he was?