Shoutbox

I think it's reasonable to suspect someone of abusing the XBL system (in terms of playing games online, uploading false achievements, downloading paid content for free, etc.) if they have made any modification to their console (considering that the MS servers can't see what you have actually done to your console).

I dont think banning such users is unfair whatsoever, the terms (I assume) state that modified units are not allowed on the service and hence you are warned beforehand. Furthermore, hardware faults are covered under warranty (which you would have to be an idiot to void by opening the unit if you are not opening it for other, less honest reasons).
RRODs are covered under warranty for 3 years and if your warranty has expired then there are alternative methods of cooling your system (not to mention that proper ventillation is sufficient in itself) than modifying the internal fans.

NOTE: This is a statement of opinion not directed at any person in particular.
...For some reason I get the feeling I should be expecting insults thrown my way anyway.