Security / Privacy concerns regarding custom android ROM What are the security and privacy concerns surrounding using a custom android ROM like that of cyanogen?
I would say the biggest issue with a lot of custom ROMs including Cyanogen's is that the OS and system apps are signed with test keys. These keys are publicly available, meaning that it is possible for someone to write an app that uses "signatureOrSystem" or "signature" level permissions. There are known cases of malware that targets these custom ROMs [1,2]. 

On a more positive note, if you run a rooted devices, you can do things like set up a host file to block ads, enable a firewall per application, selectively disable permissions, etc. Custom ROMs often ship with newer kernels, so a lot of security vulnerabilities that are kernel-based are dealt with faster than what Google can do. 

[1] http://www.cs.ncsu.edu/faculty/jiang/Fjcon/

[2] http://blog.mylookout.com/blog/2011/06/15/security-alert-malware-found-targeting-custom-roms-jsmshider/