As I understand it, iphoto will import a version of your photo from the card/camera. Then, iphoto cataloges all the pictures into an iphoto library for safekeeping.
These are typically hidden (to avoid users from messing it up) You can access by: control clicking (or right click) on the iphoto library in finder and selecting show package contents... then you navigate to you picture by date.
Now as I understand your question, you're wanting to save space and delete copies... I think the only time you get a copy in photo is when you edit a picture in iphoto...it will make an edited copy and keep the original (this depends on your version of iphoto too...I'm commenting on '08) This process can be a bit enlarging on your HDD, but I would personally hesitate to mess around in the iphoto "hidden" catalogue.... and personally I love having a few copies of pictures here and there.
Now, if you have a bunch of photos that you copied straight off the camera/card - say, onto your desktop. Then, you imported them into iphoto - you could delete the ones on the desktop.
I use time machine on a seperete HD, burn back up disks, use Iphoto, aperture, iviewmedia pro. The last thing I want, is to lose photos of things, people, places I can never view or see again!!!
On a side topic, I would also recommend printing a copy of every picture you never want to lose. Digital failure is VERY REAL! Many people don't realise that CD's and HDD's have life span anywhere from 5 years to 25 years.... depending on several factors: light exposure, humidity, quality of manufacture, magnetism, etc.
And I'm not a nerd...geeky maybe.
