UNO with SIM900 Quad-band GSM/GPRS Shield & a Ethernet Shield W5100 Micro-SD

asterof:
So is that a problem or do I need to get flexible jumpers to connect those two

I don't believe it is. There is nothing to stop you putting the Ethernet on first, indeed I'm sure it is common practice by virtue of the ICSP pins. What you do need to check is how you power the combination. I am still trying to get that sorted.

http://forum.arduino.cc/index.php?topic=253081.msg1791761#msg1791761