I would like to test Modest in my N800 device however, I can't find it in maemo download/internet or maemo diablo repositories.

Do you know where is it available?

[update:]
i found the next package using http://www.gronmayer.com/it ;
it is in a devel repository:

http://repository.maemo.org/extras-devel
Package:
modest-l10n-enus1 (v. 0.1.1)

Could be this package the latest version for diablo/OS2008?

iirc, the last available version for maemo4/diablo was included with the ssu. In other words, its not available on its own.