Sometimes I do miss the whole 3e named bonuses thing. I have had a bit of a hunt and I think that Mage Armour and Shield stack with each other but I can't find a clear ruling. The closest is this which seems to make a differentation between setting your AC (like Mage Armour does) and a bonus to your AC (like Shield).
Is that right? I guess it's not going to matter too much, and you can argue about the value of using two spell slots to avoid one hit but still good to know! 
Yes, that is correct. Mage Armor stacks with the shield spell, the same way that shields stack with armor. There are three ways of modifying your AC, (although one of them only occurs in one specific spell.)
1. An item, ability, or spell gives you a way to calculate your AC. Examples include armor, unarmed defense, and mage armor. These abilities never stack with each other. So, for example, if you cast Mage Armor on a Monk with the Unarmored Defense ability, the player can either calculate their AC as 10+Dex+Wis (as per Unarmored Defense)
or as 13+Dex (as per Mage Armor), not as 13+Dex+Wis.
2. An item, ability, or spell gives you a direct bonus to your AC. These bonuses stack with each other and with all forms of calculating your AC, unless otherwise specified. So, for example, a character wearing chain mail calculates their AC as 16, and a shield adds +2, for a total of 18. If that character has half cover, they add an additional +2 to their AC for a total of 20.
3. The Barkskin spell prevents the subject’s AC from being less than 16. You calculate your AC as normal, and if the result is less than 16, the spell sets it to 16. So, a character with 14 Dex wearing hide armor calculates their AC as 12+Dex (up to +2)=14. If that character casts Barkskin on herself, 14<16, so her AC becomes 16. If sne equips a shield, they add +2 to their normal means of calculating AC. 12+Dex (up to +2)+2=16, and 16 is not less than 16, so her AC remains 16. If she then gains half cover, she adds +2 to her normal means of calculating AC, so 12+Dex (up to +2)+2+2=18, and 18 is not less than 16, so her AC remains 18.
