SQL Plus 10g

Connecting as SYSDBA in version 9 (or 8) always requires the use of quotes around the connection string, otherwise you get

C:\>sqlplus sys/sys as sysdba
Usage: SQLPLUS [ [] [] [] ]
where  ::= -H | -V | [ [-L] [-M ] [-R ] [-S] ]
        ::= [/][@] | / | /NOLOG
        ::= @|[.] [ ...]
        "-H" displays the SQL*Plus version banner and usage syntax
        "-V" displays the SQL*Plus version banner
        "-L" attempts log on just once
        "-M " uses HTML markup options 
        "-R " uses restricted mode 
        "-S" uses silent mode

Finally this restriction is lifted in version 10.  The quotes are no longer required.