MediaTek is one of the world's biggest chipset manufacturers, and the company is basically eating up Nokia's market share on low-end (feature) phones. Opera and MediaTek already have an existing relationship, but this new agreement makes it easier than ever for phone manufacturers using MediaTek's solutions to install a proper mobile browser.
Unlike Nokia, MediaTek doesn't actually make its own phones. Rather, it enables other companies to use their chip solution to build mobile phones and other electronic devices.
Some of you may recall that Nokia has decided to go exclusively with its own browsers from now on, which means that they will no longer preinstall Opera or other third-party browsers on their devices.
With MediaTek eating larger and larger parts of Nokia's pie in the feature phone market, one can probably understand why we aren't too worried about what Nokia decides to do, its is market is vanishing on both the low and the high end (Windows Mobile 7 apparently sold fewer units in the 3rd quarter of 2011 than the 3rd quarter of 2010).
As for Opera, this new deal with MediaTek will certainly help us towards our goal of 500 million active users by 2013.