Not enough info.
How long does the battery need to last? Does the Arduino need to be able to send text messages DIRECTLY via GSM? Or can it send through some other fairly local network device? Must it be completely wireless (usually the implication with a battery, but not always), or can it plug into ethernet? Or even better, a USB port on something else close by (unlikely, but gotta ask)?
Are you talking about doing it "as small as possible" but still use fairly "plug and play" modules like existing Arduino boards and shields, or are you willing to do wiring and/or make custom boards?
What kind of "tilt sensor"? Do you need to know if it moves anywhere from one degree to 90? Or just know if it tilts past a certain point? Do you need to know how much it tilted via the text message, or just that it tilted at all?
You just can't expect people to spec your project for you.
--Donnie