What do you think of P22 London Underground? (I have no experience, sorry, if this is not appropriate)

I think its brilliant, and especially with the pro version with the lighter weights is very flexible :)
http://www.youworkforthem.com/font/T0515/p22-underground-pro-set

You could also try:
- ITC Johnston
- ITC Kabel
- Dina
- Dee Dee
- Agenda
- Granby

At TypeCon in New Orleans, Onur Yazıcıgil showed
a font by a Turkish designer which to me was most
significant because of its diamond tittles, which evoke
the Arabic past of Turkish writing.

hhp

you probably mean something more subtle than this hrant?

:-)
IIRC it was essentially a contemporary sans with some subtle
extra flavor. I'm in contact with Onur* so let me see about it...

* Concerning the tantalizing ISType-2012.

hhp

Meran.

So the good news is Onur kindly sent me some images.
The bad news is my memory was faulty - although pretty
cool it's not something that can be use for much text.

BTW, this is the designer:
http://www.barincilt.com.tr/eng/eminbarin.html

hhp

See Jante Antiqua by Danish type designer Poul Søgren.
http://www.fonts.com/findfonts/detail.htm?productid=215682