If your video editing software is able to import .MTS clips directly then just copy the STREAM folder on your SDHC card to your computer and rename the folder. Give it an unique name (e.g. event_date). Then you can burn up to half an hour of AVCHD clip folders on a DVD-ROM or up to one hour on a double-sided one and store it away.
iMovie 08 and Final Cut Express 4 don't import .MTS files directly. They import from a supported camcorder, the SDHC card, a mounted disk image or a DVD-ROM. In order to be able to re-import the HF100 clips you must backup the entire BDMV folder of each card. This folder contains the STREAM folder as well as thumbnail and clip info folders that are needed by the import window. To backup the videos on a SDHC card just copy the BDMV folder to an unique named folder on your computer.
To be able to re-import into iMovie you can burn the BDMV folder to a accordingly labeled DVD-ROM. iMovie 08 will recognize a mounted DVD as shown below and automatically open its AVCHD import window.
The second possibility is to use a disk image. Open Disk Utility and go to File > New > New Blank Image... Choose a volume size thats large enough for your BDMV folder and click OK.
Copy the BDMV folder to the mounted disk image and start iMovie 08. iMovie 08 will recognize a disk image as shown below and automatically open its AVCHD import window. You can also archive such disk images.
Another option is to make a disk image of the entire SDHC card using Disk Utility. To do this open Disk Utility, click on the SDHC card then go to File > New > New Image from ... An exact copy of your SDHC card will be saved to your boot drive. Archive, mount and import such an image as explained above. The only disadvantage of this method is that the disk image size is always the size of the SDHC card even if it's half full.
I've tested this with iMovie 08 version 7.1.1 but I strongly recommend that you test it too on your computer before relying on it.
UPDATE: If you have to backup a 16 GB SDHC card to standard DVDs move .mts files that do not fit on the first DVD into DVD2, DVD3 and DVD4 folders and burn just the .mts clips to additional DVDs (I'm assuming that no single clip is longer than 30 minutes e.g. <4.7 GB).
To restore the card copy "DVD1" with the original folder structure to the harddisk then fill the STREAM folder with the missing .mts clips from DVD 2, 3 and 4.
For import into iMovie '08 use "Make image from folder" in Disk Utility and mount it. I don't know if Final Cut Express 4 needs a mounted image but Final Cut Pro 6 imports just fine from a folder with the original structure.