A mobile version of Mozilla has always been coming, likely in 2010, and the announcement of an 'alpha version in a few weeks' probably fits into this time line, albeit a conservative time line.
There's no word yet on which platforms this will be targeted at, but it's likely going to be fully Open Source, and I think that once the world gets their hands on the code, we could see it implemented on one of the popular open platforms. To be honest I think many will consider this browser is going to be pointed at the Google Android platform, but all it would take is one or two Symbian coders (or Windows Mobile, or Apple, etc) to spend some time with the source code to have a native version.
Do we need another major browser though? To be honest I think any healthy ecosystem always needs two players to help drive innovation between each other, and to keep each on their toes (eg with bug fixes and compatibility with new website technology), so I look forward to Mozilla getting involved. And with more recognition of mobile web access, that's going to help the whole web ecosystem to remember that mobile is a growing audience.