would be a quite arbitrary and hardly meaningful comparison, wouldn't it? the original iphone, when it was introduced, was a very simple, low-complexity, no-frills feature phone that had the whole companies marketing machine behind it - and carrier subsidies.

the N900 is a very powerful, complex, hardly subsidised mini-computer that enjoys very little marketing only and has a company behind it which (rightfully) says: we didn't make this for the mass market, think twice before you buy it.

as i said before: the benchmark for the N900's success aren't the iPhone or the Nokia C3 or the Samsung Corby; not even so-called smartphones. the only valid banchmark is Nokia's own expectation about the commercial success.
don't forget it wasn't designed to be "as successful as possible" and to be sold to "every consumer on earth, if possible". it was designed for a certain type of consumer. and it may be a surprising finding that it either appeals to other types of consumers as well... or the target group is just larger than expected.