First, the thing you need to remember about noise is that it is only indirectly related to ISO. The real culprit where noise is concerned is a low Signal to Noise Ratio (SNR). This is technically answered using controlled testing to illustrate the point in this question: Is high ISO useful for photography? This answer to another question addresses the more general issues regarding image noise in a succinct way.
As an owner of one, I have often been frustrated with the amount of image noise produced in low light situations by my Canon EOS 7D. Based on reviews I had read before transitioning from a 50D to the 7D for my "long" body I was expecting about a 1 stop improvement in high ISO noise performance. The 7D is a more advanced camera in many ways compared to the 50D, especially in terms of handling speed and the configurability of the Auto Focus system. But I have not experienced much, if any, improvement in terms of high ISO noise performance. As the answers in the questions linked in the paragraph above indicate, how much noise appears in an image is determined by a variety of factors, not just the selected ISO. In a well lit scene you can shoot at a higher ISO than in a poorly lit one before you reach the point that the SNR becomes low enough that the noise is noticeable.
This image illustrates that even at ISO 3200, if the subject is well lit it can be shot at high ISO without a lot of noticeable noise. Since the background was almost totally dark, it was very simple to reduce the exposure of the shadows to deal with the noise in the dark area surrounding the subject. The singer was illuminated by at least one high intensity spotlight as well as colored stage lighting mounted above and below the front of the stage as well as above the back of the stage. ISO 3,200, 1/1,600 second @ f/2.8.
On the other hand, further back on the same stage, but without the benefit of the spotlights directly on the subject, this photo shot at the same ISO for twice as long shows more noise. ISO 3,200, 1/800 second @ f/2.8
So what does all of this mean in terms of shooting a wedding with the Canon EOS 7D?
- It means use whatever ISO necessary to take the shots you need. If you drop the ISO and lengthen the shutter speed (Tv) to increase the amount of light that enters the camera, you also risk allowing subject or camera motion to cause blurring. There is no good post-processing cure for motion blur. Up to a point there are ways to ameliorate the amount of noise in an image.
- It means learning to use the available methods of reducing noise during your post-processing workflow. Often the best tools in terms of handling noise during the RAW conversion process are provided by the manufacturer of the camera, probably due to the manufacturer's intimate understanding of the sensor's design as well as access to the often proprietary algorithms used to record the RAW output from the camera's sensor. Third party tools, such as Lightroom 4 and more recently LR5 are catching up, though. There are also a number of very useful tools, such as Noise Ninja, that do an excellent job dealing with image noise after an image has been converted to formats such as JPEG, PNG, Bitmap, etc.
- It means considering using a camera body with a full frame sensor for shooting in low light weddings. Although not cheap, the larger sensor and the resulting increase in the size of each pixel when comparing a FF sensor to an APS-C sensor of similar resolution means about a one stop improvement in light gathering ability between the APS-C 7D and the Full frame models such as the 5D II or the 1Ds III. Newer models, such as the 5D III, the 6D, or the 1D X also benefit somewhat from improvements in sensor technology. If you are shooting RAW any improvements in Canon's or third party's RAW convertors can also be applied to image produced in the older bodies. Yes, a FF body is costly but so is producing work that is not up to the highest standards. At least in the region of the world where I live in the Southeastern U.S., the most significant and effective marketing tool for a wedding photographer is a growing list of satisfied customers who recommend their photographer to friends and family members when they are looking for a photographer for their wedding. And satisfying your wedding clients often results in follow up business for baby/children/family portraits further down the road.