How can I get major version updates with drush pm-update?
You can explicitly ask Drush to tell you all of the available releases for a specific project using the pm-releases command:

$ drush pm-releases mollom
------- RELEASES FOR 'MOLLOM' PROJECT -------
 Release         Date         Status                 
 7.x-2.x-dev     2012-Dec-19  Development            
 7.x-2.3         2012-Oct-22  Supported, Recommended 
 7.x-1.x-dev     2011-Dec-20  Development            
 7.x-1.1         2011-Jul-09  Supported, Installed   


However, what you want to do -- to have pm-update(code) automatically notify you when there is a newer major release -- is not supported.  I have opened a feature request in the Drush issue queue for you:  http://drupal.org/node/1880970

UPDATE:

Regarding kiamlaluno's comment, you can force the update using pm-download:

$ drush dl mollom-7.x-2.3
$ drush updatedb


I think that it would be more difficult to write a script to check for updates with pm-releases than it would be to enhance pm-updatecode to show this information.  If you'd like to help out, please post a patch at the issue I linked to above.