a few things I'll have to disagree with you on.
1) I never questioned the viability of the platform - this is definitely something that can be made. I do question the viability of it as a business if you are relying ONLY on ad revenue. However, adding additional revenue streams will increase the value and possibly serve your market better than advertising. My initial reaction is not 'this is going to be a star', but you could have a viable business here, I'm not your target market, and can't relate to what you are doing. You probably don't read mtbr.com or pinkbike.com, I do, you might not have thought those to be viable businesses, but they are.
2) I'd suggest re-reading your paragraph which starts 'My reasoning...'. firstly, who reads reviews for something they aren't keenly interested in? and the only people I know who have iPhones have them, but use them just like any other phone, meaning they don't install apps, or read about iPhones etc.. I have a blackberry and don't install applications, I don't look at blackberry focused sites AT ALL, and I am a geek!
secondly, you can't just take the total number of phones and say that is the potential market. Everybody uses toilet paper, but the market opportunity is not 9 billion people.
iPhone and Android are not the first players in this market. I'd take a closer look at sites that do what you are thinking of doing for Windows Mobile, Blackberry and Symbian devices (even though you have identified differentiators, look at what has been done to help define opportunity size). These devices already have a larger user base with 118 million devices sold in '07 so 10 times larger than iPhone currently.
Also, don't be fooled by Apple marketing/PR and the way the numbers read. 10 million devices sold is more like 7 million users. I had gone through 3 iPods before most people had bought their first one, fanboys buy the newest thing they can get. Also, the planned obsolescence of mobile devices is 18 months, so when they say '100 million sold in two years', that likely means 50 million users. It's not how many devices HAVE been sold that matters, it how many devices came onto the market in the year (or 1.5 years) that is the important figure in mobile (I've done some mobile consulting).
So i actually just tried this with Blackberry and Symbian, and interestingly, the largest sites I found (quick search) has about 40k montly visits. Similar to the size of the top iPhone reviews site. This suggests to me that opportunity size is in the 30-80k range.
Something to think about...are more people going to use read/write reviews BECAUSE your site is better? or are you going to be getting users from other sites because your site is better. It's an important difference, and will help estimate market size.
For my site, I postulate that finding live music and deciding to go see a concert is a poor experience, but that if people had a better way to find out who's playing, sample the music, etc. they would go see more shows.
So for my site, I say 'with a better experience more people on a whole will use an online tool to accomplish this task'.
Rather than saying 'I'll be able to grab x amount of users from other sites which do similar things, but don't offer as good an experience'.
You've defined the opportunity where current sites are missing the boat, so that's good. You've got a target you are aiming for. You'll find more opportunities as you dig into this I'm sure. You'll also find challenges to things like categorization and taxonomy - who knows, maybe you've already got a solution to these challenges. What I think is interesting is that I suspect that as you go down this road, you may find greater opportunities than you are originally planning. Categorization can be VERY challenging, it is something I am struggling with on my site (your electronic music example is perfect!). So if you have identified a method for managing categorization, maybe the app review site is just an opportunity to flush out managing categorization, and maybe that will be your long term business.
I can't answer your last question about 'will it be enough'? Only you and your partner can decide that. If you have the ability, I would suggest building something and getting it out there while hanging on to your day job.
Building is often the easy part, the challenges can come in actually getting people to use your site.
If you build something great, maybe you can make a living, maybe you can get investors involved. If you don't build anything, you likely won't get either.
Costs of developing something like this should be very low (hopefully you will code it up yourself). This would probably be a good opportunity to try something like google app engine.