Stick to the market and do not download apps from untrusted sources,
Same goes for any system,
Ever have a program ask for admin privledges to make changes to this computer on Windows? Same thing, except on Android that data the apps get access to is divided so that apps don't gein universal privlages to just access everything, they have to specify exactly what info they need.
Android is very paranoid, which is why I hate it. I can't even hack my own phone let alone someone elses. It would take some serious social engineering and redirection of web traffic to get them to download a malicious app. Alot of work unless you put them up like all those free apks that u usually have to pay for are probably some form of malware in em.
"Wise man say, 'Forgiveness is divine,
but never pay full price for late pizza'."
Some Usefull links:
// Good read on the Android specificities of the kernel
// Info on Binder and ashmem and how to share memory!