I am posting this in the hope that someone may have run into this previously, and they can say "oh!! you just need to do such-and-such..." I will keep looking into this in the meantime till I work out what what I did...
I was creating my very first view, with the views module. I selected the "add" tab, and filled in the details on the initial fields, like title, URL etc. I selected the checkbox on the "page" section, and I also selected the "block" section as well. I did not add ANYTHING else - I left all the other fields blank, which may seem strange, but, there you go...
I published the block to one of my sidebars, and the the site totally barfed and would not load (ever again...). I reloaded from a previous backup, and tried to reproduce the problem.
If I simply click the "page" box, and don't publish any block, and then go to the URL, I get the same error that I got before, namely:
Fatal error: Cannot redeclare dateformat() (previously declared in /home/...../public_html/includes/common.inc(1347) : eval()'d code:8) in /home/...../public_html/includes/common.inc(1347) : eval()'d code on line 6
I have tested this same thing on a clean Drupal install, and I dont get the error, so obviously its a module or such thing which is not playing nicely. I will try and isolate this by gradually turning off all my modules, but it sure scared me that I was able to totally trash my site simply by creating a view... backups are nice :)
Anyway, if any fellow G&G drupalers reading this have seen the same thing, and the solution is "obvious", please let me know, thanks!