The i8910 runs Symbian which is a real smartphone OS. As a result, it has a version of coreplayer and other apps which can decode avi, divx, xvid, etc even though the manufacturer might not have made such an application available.
There is no coreplayer for android because android apps are not native -- they run in an interpreted Java VM (it doesn't even have a JIT). In fact, I personally classify Android as a dumbphone OS -- there have been dumbphones that can multitask J2ME apps for years, and if all of the JSRs are implemented, J2ME is a rather complete spec (it even has accelerated 3D, something that Google is just now getting around to). To me, the defining feature of a smartphone is the capability to run native apps ... so both Android and Blackberry are just really, really fancy dumbphones.
Granted, Google has released the "NDK" (native development kit), but the programmers behind coreplayer have declared the NDK to be a complete joke, and have stated that they will not be bringing coreplayer to android until Google more fully addresses the lack of a way to interact with the phone hardware on a lower level.
I love my Nexus One, but IMO this is one area where google really really dropped the ball. Windows Mobile, iPhone, S60, Maemo, and soon even web OS support native apps. Google needs to do two things:
1) Get its game together and offer real native app support
2) Offer APIs to hardware accelerate multimedia functions (I think they may have finally added 3D in 2.1)
... I'm sick of having apps on my 1GHz snapdragon that look like gameboy advance crapware.
If I'm wrong about any of this stuff, someone please let me know, but as far as I know this is the case.