There is intrinsic security and there is perceived security. Regarding the former, I absolutely agree with reinob. Maemo is intrinsically insecure for many reasons, not just because of rootsh. You have no way of knowing or controlling what the apps you install do behind your back (unless you check their source code, but who can do/does it?)

Android is more secure since you have to explicitly allow applications to do certain things. The trouble is you have to do it at install time and you have to allow all or nothing. No way of saying, I am happy to install this cool game but I do not want it to connect to some server in China. Which sort of defeats the whole point and what is meant to be a security measure becomes just another annoying step that users just want to wave out of their way.

I do not know enough about iOS, WP or RIP to comment. Sticking to what I know, PalmOS was just as bad as Maemo with its "free for all" approach, with the added benefit of the source generally not available. Symbian was probably the best in this regard but, as has been mentioned before, it went a bit too far to the point of being intrusive, both for the user and for the developer.

Then there is the perceived security.

This is a bit trickier and depends on many factors, the most important of which is the OS's popularity. Despite various claims of open-source fascists enthusiasts I do not agree that on the desktop, Linux is intrinsically more secure than Windows. I would even argue that it is the opposite. Windows allows you a more fine-grained level of specifying who can do what and has a more focused development force behind it. But it has two big disadvantages. One, it is the most common (popular?) platform and two, partly because of number one, it is used by clueless users. Both make it a popular target for attackers. Security holes are more likely found and more eagerly exploited.

In this regard, Maemo is probably one of the most secure OSes simply because no one will bother to attack it and Android the least secure because everybody will. To illustrate it simply, I would not bother with any security software on Maemo but it was always my first thing on Android.