upgrading to innodb plugin

As of MySQL 5.1.38, the InnoDB Plugin is included in MySQL 5.1 releases, in addition to the built-in version of InnoDB that has been included in previous releases. So all we need to do is just enable it, that is, add following options into my.cnf file

[mysqld]
..
ignore-builtin-innodb
plugin-load=innodb=ha_innodb_plugin.so

for further readings visit here http://dev.mysql.com/doc/refman/5.1/en/news-5-1-38.html

Assuming you have succesfully enabled Innodb Plugin, now what? would you automatically get benefits from all new Innodb plugin features?
The answer is yes/no, because Innodb Plugin supports new file format 'Barracuda', which introduces two such new data structures: compressed
, and long variable-length columns stored off-page. By default, the InnoDB Plugin does not create tables in a format that is incompatible with the built-in InnoDB in MySQL.
This means you won't get these two features automatically until you create new tables using new file format or convert exisiting tables to new format.

Innodb Plugin offers lots of other features too and these features are enabled by default, also you will automatically get benefit from these features such as fast index creation and lots of performance and scalability enhancments,
for full details click here
here http://www.innodb.com/doc/innodb_plugin-1.0/innodb-performance.html