PS C:\Users\Administrator> $w = Get-SPWeb -Identity http://demolab-sps2010/sites/atest
PS C:\Users\Administrator> $l = $w.Lists["Tasks"]
PS C:\Users\Administrator> $f = $l.Fields["Status"]
PS C:\Users\Administrator> $f.DefaultValue = "Completed"
PS C:\Users\Administrator> $f.Update(true)
Missing ')' in method call.
At line:1 char:11
+ $f.Update( <<<< true)
    + CategoryInfo          : ParserError: (CloseParenToken:TokenId) [], Paren
   tContainsErrorRecordException
    + FullyQualifiedErrorId : MissingEndParenthesisInMethodCall

PS C:\Users\Administrator> $f.Update("true")
PS C:\Users\Administrator>