Thanks Erik & Jeff for your answers :)
Jeff can you please explain why this is considered good programming? To me it makes skinning harder (as only some id's are prefixed with dnn_) & I really don't like the idea of the general public knowing what software is being used because all software is prone to exploits.
I know if I was trying to hack a website the first thing I would do is try to identify the software being used so I could look for expolits in it. If I found the software had not been patched to resolve known exploits I would then try to take advantage of the exploit.
One good example of this was when Joomla 1.5 had a serious exploit & everyones Joomla website got hacked if they were not smart enough to patch it straight away - see: http://milw0rm.com/exploits/6234
Now I have to also say that DNN has a great security policy where they do not talk publicly about known exploits, but as most people saw/read not so long ago sometimes people can do the wrong thing & release details of the exploit to the public. Now if I were running older versions of DNN I would be worried that people could find my unpatched DNN site (just like they did with Joomla sites not so long ago) - they could then take advantage of any previous exploits that they know about.
I have also checked these sites & they seem to limit branding thier code:
http://www.umbraco.org
http://www.kentico.com
http://www.ektron.com
http://www.adxstudio.com
http://www.sitefinity.com
I know DNN takes their security very seriously (thats why we use it) but would be interested to know other peoples thoughts on this matter are.