The first error (about the locale) is because you upgraded to Symfony master without reading the UPGRADE file describing the needed change.
the second error is because you did not rebuild the bootstrap.php.cache file after this upgrade and so you are still using the old version of some of the classes whereas the other ones are using the new methods

Seems like you are using a really old version of Symfony. The MapFileClassLoader has been replaced by a MapClassLoader during the beta. And the uptodate build_bootstrap.php script is now in SensioDistributionBundle, not in bin/

I've downloaded the lastest versions of symfony without vendors, and installed the vendors via terminal,
I've run php app/bootstrap.php.cache,
and run php app/console doctrine:mapping:convert xml ./src/Acme/BlogBundle/Resources/config/doctrine/metadata/orm --from-database --force

And i still get this error for trying to introspect an existing database and create mapping information

 [Doctrine\DBAL\DBALException]
   Unknown database type  requested, Doctrine\DBAL\Platforms\PostgreSqlPlatform may not support it.

What's the right command for rebuilding the app/bootstrap.php.cache???
Is it necesary to select a schema somewhere???

Appreciate your answer...

The unknown type is an empty type here (see the 2 spaces with the type between them). This is a bug in Doctrine DBAL regarding PostgreSql. It may have been fixed in the DBAL master branch (which will probably require using the master branch of Common and ORM as well) as it has been reported a while ago but not sure.
@beberlei can you confirm ? I saw the discussion on IRC but I haven't found the ticket for this on JIRA.

So i've updated Doctrine DBAL master branch, Common and ORM.
I've rerun

 php app/bootstrap.php.cache

and run

 @:/home/Symfony2# php app/console doctrine:mapping:import "AcmeTaskBundle" xml --em=default --force

but I still get the same error

 [Doctrine\DBAL\DBALException]
    Unknown database type  requested, Doctrine\DBAL\Platforms\PostgreSqlPlatform may not support it.

What could be the problem???

Can you ask for support on the Symfony mailing-list? Thanks.

The problem is Doctrine's... It doesn't have compatibility with postgres9.1, but with postgres8.4.
Installed 8.4. Worked. And then another problem with Symfony/Doctrine and schemas in postgres came up.

@ilbp90 Can you open a ticket in the Doctrine issue tracker about the Postgres9.1 support ?

Hello,

Please anybody reply for this error Unrecognized options "default_locale" under "framework.session"

Thanks & Best Regards,

J Ajay.

Read the UPGRADE file. The default locale is not a session setting anymore since 2.1 as the locale is not managed by the session anymore.

Thanks! stof,

But When I setLocale with $request->setLocale('fi') and $request->getSession()->set('_locale', 'fi'), is not getting effected,

Should I comment fallback in config.yml? and other action need to do.. please advise.

Thanks & Best Regards,

J Ajay.