I personally have never used Illustrator but I am pretty sure its just like photoshop, if so then you can just make an alpha channel in the channels tab of the layers box and then use the wand tool to select the desired parts (the letters) and make them black and the rest white in the alpha channel/layer. This will then make the white part around the text transparent (as long as your saving out in a format that stores alpha (tiff, targa,etc).

Hope this makes sense as like I said I haven't actually used Illustrator.

Chris