a "mobile" startup is a company concentrating on "mobility." There's a lot of opportunity there. And also, the fact that most phones suck is an opportunity. Apple is cashing in on that by making a better phone. You can cash in on that by making the existing phones suck less with your app.
By definition, making an existing phone suck less is a mobile startup opportunity that involves neglecting the iphone.
iPhone is a medicore phone and a sub-par email device.
It's an ok line-of-business platform, but subpar to competition.
It's a beatuful web device, a great multimedia platform and decent entry-level gaimng platofrm that grownups can carry around unashamed (a "suit" will not carry PSP, but iPhone is Ok). It's also a fashion gadget and a status symbol, especially aborad.
So, depending on who your customers are...
There is a lot of room in the mobile space but my biggest fear is that instead of developing substantive companies, businesses pander on the spam/one-hit-wonder line. Don't be one of those companies, spend the time to develop an idea that has value and meaning.
And Android is like the linux for phone OSes (which is a weird thing to say since it actually is linux..), consumers want a unified, polished experience, not an ad-hoc one.