Is the smartphone still a kind of node? I mean, does the smartphone add anything to the 'security' of nxt when it is running a lightweight client?
No.
We conclude: This forging smartphone marketing thing is kinda bs. I could state that my penis is forging too, because I leased my forging power. The only difference, for example to BTC, is, that we can run a lightweight working client on our smartphones to make transactions, without the forging (and thus network securing) function.
Bitcoin is at 7 TPS ...smartphones can easily handle that....I think high end smartphones (Note 3 or Iphone 5S) can easily handle 1000 TPS....if you have fast internet.