I now creating the current release of the severino. On the schematics the C9 capacitor is polarized, but on the mounting diagram the capacitor's mounting direction is not indicated, it's shown as a non-polarized capacitor. This is the last part of the board assembly, plz help.
Sry for the bad english, I'm working on it. 
Well well, there is a problem on the http://arduino.cc/en/Main/ArduinoBoardSerialSingleSided3 part of the arduino site. The link under the picture, that points to Schematic (.png) is wrong, but in the pdf file the C9 capacitor is shown correct on the schematic. Well, the question is now answered. 
Hi,
You are right. There is an inconsistency between the webpage schematics and the .pdf manual. The final project consider C9 as a non polarized capacitor, as described in the .pdf manual.
The webpage schematic shows C9 as a polarized capacitor, with the plus (+) side to the GND. This is not wrong, it's an alternative, and you can make the board using this schematics, if you can't find a non polarized cap to buy.
Is this make a difference? I mean will there be any disadvantage if I use it like that, with the polarized capacitor?
Can I use two polarized caps connected their + to each other instead of a non-polarized cap?
I don't know if is there any disadvantage, but in theory, the cap will work with inverted voltage sometimes, reducing it's life. But I don't think this is a big problem.
A guy here said he used the trick of soldering + side of two polarized caps. I don't remember, but I think it didn't work. I think a polarized cap is fine.
hey there Adilson,
then, can we use 10uF ceramic / polyester / ceramic disc capacitor?
instead of electrolytic ones.
i am now connecting my the +ve to the GND, like you said for the C9 10uF capacitor, using a polarized electroytic capacitor.
i'm going to do some tests to see if the board works.
hopefully only the capacitor would fry, not the board. :-/
thanks for your reply.