|The xbox itself is extremely easy to open and repair if you have the Red Ring of Death, however as Phil has said if you have warrenty it will be null and void. |
Opening your xbox will not mark your account in any way, However there are many people that apply 'hacks' 'mods' etc to their account which allow them to change their gamer score and other such things. If you get caught doing this then your account could be blocked.
I know a few friends whos xboxs have been banned from xbox live for violating terms and conditions, ie modding their accounts.