I don't think there is any way to transfer an event to Lightroom. I've only dabbled a bit with iPhoto but I use Lightroom regularly so I'll take a swag at helping you.
First, set your Mac so iPhoto does not launch when you download photos. Launch Image Capture (it's in your Applications folder) and insert an SD card or CF card into your computer. Select the device and then select No Application under the Connecting This (Device) Opens tab. This will allow you to copy the photos using the Finder and give you the ability to decide where to store them.
Next, launch iPhoto and click on Photos under the Library heading in the left column and select the photos you want to work with in Lightroom. Select File: Export. In the next dialog box box select Kind: Original and File Name: Use filename. Click the Export button.
Now navigate to where you want to store your photos. Create a new folder and give it an appropriate name. Click OK. If you have a lot of photos do something else while iPhoto works on the images.
You will now have copies of your photos in a new location and can launch Lightroom to work on them. When you are absolutely certain you have all your photos in your new location go back to iPhoto and reselect the photos as above, then choose Edit: Move to Trash. This
should move the selected photos to the trash, but did not work for me when I just tried it. I went to the Finder and manually trashed the iPhoto Library folder. Do not do this unless you are certain you have all your photos moved and backed up.
I wasn't worried about trashing the iPhoto Library folder because I do not use iPhoto. You may want to search the 'net for ramifications of trashing this folder if you might use iPhoto in the future. I accept no responsibility for what happens to iPhoto's database.