Playstation Network (PSN)?

May be...

Whilst i'm logged into the 'My Playstation Network' ONLINE via the Internet, can my friends see my online stautus via the PS3 console? (and does it also effect the 'Last login time' displayed on the PS3 console)?

They can not see you.

They can only see you when you are connected with the PS3.

Last login time is also PS3 only.

if you have entered your details of your PSN account into the website then they probably would be able to see ur online status. i havent tried it for the ps3 but when i used to have the xbox 360 i used to do instant messaging all the time on the computer via xbox live account.