Another answer I posted to the NG, and not all that hard once you know the right classes to use from the BCL. But if you didn't know where to look, I can imagine it being a royal pain in the ass.

--- begin ConvertTo-Sid.ps1 ---

param ($account = $(throw "need account in form domain\username or
[ntaccount] object"))

if ($account -is [security.principal.ntaccount]) { 
    $ntaccount = $account

} else {
    $ntaccount = new-object security.principal.ntaccount $account

}

$ntaccount.translate( [security.principal.securityidentifier] )

param ($sid = $(throw "need sid string or [securityidentifier] object"))

if ($sid -is [security.principal.securityidentifier]) {
    $securityidentifier  = $sid

} else { 
    $securityidentifier  = new-object security.principal.securityidentifier $sid

}

$securityidentifier.translate( [security.principal.ntaccount] )

--- end ConvertTo-NTAccount.ps1 ---

You can pass strings as args, or their respective native objects. They both output objects. The output of one can be used as the input of the other.