Abshishek reminded me of this with his post on how to figure out Security Group membership using PowerShell. But Distribution Groups are a whole different beast. So how do you do it? One way is to use the GUI: the awesome Exchange Management Console already shows you group membership. And on the cmdline you can do this in Exchange 2007:
Using server side filtering (faster):
$filterid = (get-user Administrator).Identity get-group -filter { Members -eq $filterid }
Using client side filtering (slower):
get-group | where { $_.Members -like '*Admin*' }
This can be adapted to do nested membership as well---I'll leave it to you to figure out how.