In addition to what the others have been saying (backups, batteries, etc.), I second the advice to get a 50 mm f/1.4. A 135 mm f/2 on a crop body would already be
very long, but perhaps useful in church during the ceremony. I used a 80-200 mm f/2.8 and the extra stop is going to help. Make sure to familiarize yourself with the `architecture' (since you're outside). In my case, there was a big column just at the right distance to take all the money shots at 150~200 mm. A tripod/monopod is extremely helpful. Since you didn't mention you had one, I suggest you rent/buy one.
I'd also suggest you tell them about your experience honestly. Don't be fooled into a false sense of security here: before they ceremony cheaper is better, but if you screw up (and we all know you won't
), but if you do, then all this doesn't matter so much anymore. Also, talk to the couple in advance of what they except of you, what shots they'd like to have and such.
Other than that, on the day of the wedding, try to stay out of sight during the important moments (if possible). You're not a paparazzi!