The iPhone will not lose momentum to anyone, it's simply too huge a market at this point. This will be greatly reinforced once the iPhone specific devices start appearing, much as even just the audio only iPhone accessories created a huge hurdle for makers of other MP3 players to overcome against the iPod...
Not that other platforms will not grow in popularity too, it's just that they will not catch up and mobile developers will mostly target the iPhone first.
I've been developing for the iPhone since the SDK was released. I will say that at first releasing to the app store was hard almost to the point of madness - but they have cleared up a ton of the difficulty (especially related to certificates) and have made the process about as clear and as straightforward as it can be now, with documentation that is extremely clear on what you need to do because they have seen people mess up everything possible to mess up.
Over time I've seen bad reviewers and good reviewers. Here I think, Apple was simply overwhelmed for a while with the sheer number of submissions and have only started to get that aspect under control. But the are trying and often a gentle prod at developer relations (best is by phone) can free up a process that seems stuck.
In the end I enjoy the platform because I think they have a great set of frameworks and the development community is very helpful, just look at StackOverflow. I don't program to the iPhone just because of the potential market but because the potential to develop a really interesting application that many people will use and enjoy is there. In the end I'm driven a lot more by the prospect of building something others will find useful than any large reward. On the Android I feel like you can get a lot of technical users but on the iPhone if you build an app right you can get users of all kinds, and that is really exciting.
The one area where I think Apple has a good amount of work to do at this point is actually in tools to assist with the marketing of apps. The whole system is really primitive now with respect to properly tracking marketing efforts or allowing discounts and so on. But again they have built up a global distribution system that works pretty well, so now they have the luxury of a strong platform on wish to base these other needs and I expect to see them shorty.
One point of proof at the degree of effort Apple is putting forth into helping iPhone developers is the WWDC videos - last year it took about six months for the videos to come out, just as they had every year prior. This year they were out in weeks, because developers could simply not wait six months to leverage the information the videos had (especially 3.0 features)!