We do a lot of corporate meeting video support and we are trying to come up with a delivery format that makes the most sense. Our current thinking is to format at 32G usb thumb drive in exfat then either drop a wmv and a .mov file to try and cover both the mac and windows world. Keep in mind in most cases we are not dealing with video or computer savvy people. We are trying to find a one size fits all solution for those times when we cannot get client on the phone to find out what exact format is needed from their web people. I should mention we use usendit but the 2g limit makes it prohibitive sometimes. I would be curious to here what other people are doing?
The thumb drive may end up being the best thing. Thumb drives can be accidentally erased or corrupted by a user. A data DVD would be the other choice, but don't get me started on how many people can't understand the differences between CD and DVD drives and media, or a data disk vs an mpeg2 standard DVD. Did you consider a central server that you control access to, where they could click a link to download?
If you do the thumb drive, maybe you could add an executable to it that will auto-run and boot in windows with a step-by-step guide on it to quiz the user's intent, and then direct them as to what to do next. Like, remind them to copy files to their desktop and not try to run the slide show from the thumb drive.
I've had very good luck dealing with non-technical people by sending them an unpublished link or a private link via my YouTube channel. Even a monkey can play it back, and there are several free YouTube HD downloaders available which you could give to the client. It's saved a lot of hair pulling on my end, and hours of time prying the details out of the client as to why they can't play it back, or there's audio and the screen is white, or...you get it.
We often deal with companies that have little experience with digital files. Often we courier a hard drive to them (either ours that they can return or we invoice them for it) BUT you need to ensure it is compatible with mac and windows. Vimeo is a good site to use for high quality video too. I'd say it's important to educate your clients on how to access video. Perhaps have a "How To" document ready to go as an attachment with emails you send?