AcidKid and Jomo, very good posts. Just wanted to make one more: many Arab-speaking Christians use the word ‘rubb’ (usually spelled with a dumma, thereby giving a ‘u’ instead of an ‘a’) in their services when referring to “God.” It does mean ‘lord’, and is often used it would be in English-speaking Christian services.
A note: most subtitle creators (here in the ME pretty much every broadcast now has a subtitle in Arabic) use ‘rubb’ when the English word is “God”; this is common among Syrian, Lebanese, and Egyptian Christians. Among those communities, at least, there does exist a differentiation between Allah and ‘rubb’; for instance, a common utterance among Muslims is ‘wallahi’ or ‘wallahi ala’zim’, meaning roughly “By God” (this is kind of like the usage in the phrase ‘by God, I mean to do that right now’). Among Christians in the Levant and Egypt, the same meaning is often conveyed by ‘ia rubbi’ or "oh my Lord.’
Hope this adds to the already interesting discussion.