Phone + OBD2 = possible?
 

I really would like to do this:
1. my car is ISO 9141
2. my phone (SE-W580) has an UART interface
3. I would like to build a small board (very small possibly) to adapt the UART to ISO9141 (using something like this maybe MC33199 Product Summary Page)
4. program some java app to monitor and calculate the consumption (algorithm are available on this very site, great!).

Problems:
a. I don't know JAVA (but I am slowly learning it)
b. I don't know what kind of transactions are needed for ISO 9141 (but I haven't looked at this site yet, sorry I'll do).
...

But I just wanted to know if you guys think is a good idea or terrible/impossible.

It's possible. I'm fairly sure there's software out there for PDAs and such already.

tangomar -

Yes, I think you can do it. I think you might want some kind of elmscan to go between the OBDII port and the phone :

ScanTool.net - Products

Here are some (sometimes not so) freewares :

ScanTool.net - Software - Browse

CarloSW2

You might look into superwaba as a development platform, it is java and can run on a lot of different phones/pda. Look at the elm323 datasheet also for an idea of what commands are out there.