There's this feature on ARM called TrustZone, which divides the CPU in a secure and non-secure world. However, it's up to the SoC vendor to decide whether to implement it or not and AFAIK it's hard (if not impossible) to use it as an end-user without modifying the bootloader (which is in ROM).

[misposted this to 16387.html, please delete the other one | also, your OpenID coupling seems to be giving problems with the captcha validation]