Plone 2.0.5 to Plone 3.1.1 upgrade

I'd been working on upgrading our corporate website which is currently using Plone 2.0.5 to the latest version Plone 3.1.1, been googling around and reading stuffs on how to properly upgrade to the latest one. We've decided to do the upgrade because we've already missed several upgrades and wanted to avail new features and feature polishes but it turned out that upgrading from 2.0.5 to 3.1.1 is not an easy task most of the products and templates are already deprecated, new implementation of portlets, the use of viewlets in replacement to the default plone templates, acl user is upgraded, tabless skin is removed although you can use NUPlone tableless skin as an alternative, products that I am using in Plone 2.0.5 have no equivalent in Plone 3.1.1 and much more. I'll try to document as much as I can while doing the upgrade and hopefully I can post it here.