If you're pretty familiar with iPhoto, you may be aware that you can select an image and choose Photos > Hide Photo (or hit Command-L) to hide embarrassing pictures in your library without actually deleting anything.
That's awesome and all, but the problem is that your item count in the upper-right corner of iPhoto's window still shows that you've hidden some pictures.
Which is decidedly un-awesome, especially if you don't want anyone to know you've got stuff you'd rather not show. After all, the only thing someone would have to do to see your secrets is choose View > Hidden Photos. Yikes.
One way to completely hide those images, though, is to segregate them from your existing iPhoto Library into a separate one, so that someone who was poking around on your Mac would have to have some knowledge (and a lot of suspicion) to get to them. Not many people would think to select File > Switch to Library from within iPhoto, especially without a big ol' red flag like the whole “look at the number of hidden photos I have!!!!” thing. And you could even pop that new library onto an external drive or something and squirrel it away so that even that trick won't work. As always, I'm not judging you or your level of paranoia.
To get started with your new library, make sure iPhoto isn't already running, then hold down Option and click on the iPhoto icon in your Dock. You'll be presented with a dialog box asking you which library you'd like to open, but you'll click on Create New instead.
After you do that, name the library and save it somewhere. I'd suggest some incredibly obscure place if you're not putting it onto an external drive.
Then you'll need to pull the questionable images out of your original library and put them in the new one. An easy way to do this is to create a folder on your desktop, then drag and drop from iPhoto's window to that folder. You can also use iPhoto's File > Export command, which may be handier for you if you've added keywords, location info, and so on to the photos you're exporting, as that way you can retain that data.
You can also use this to export an entire album, so if it's more your style to drag the offending photos to an album instead of to your desktop, do that and then use the File > Export command.
OK, so you've got the photos out. Now open up your new iPhoto Library by either double-clicking its file or by holding down Option again when you launch iPhoto to pick it from the list.
Then you can simply drag the files into the iPhoto window (as suggested in the screenshot below) or choose File > Import to Library to do the same thing.
When all the photos are imported, you can delete them from your original library. First, though, check to make sure iPhoto > Preferences > Advanced > Copy items to the iPhoto Library is on, as that'll mean the actual files have been copied and you won't lose anything. If that's on, you should be safe to get rid of the old ones. Don't forget to choose iPhoto > Empty iPhoto Trash afterward. And THEN (oh my GOD Apple) the images get put in your Mac's Trash, so clear that out, too. And the stuff on your desktop if you put things there in the interim. Geez Louise! It's tough having stuff to hide.
From then on, you can choose which library to open by just doing the old holding-down-Option trick as you launch iPhoto or by using the File > Switch to Library menu item.
Two cautions: First, be careful if you've made edits to the images you're moving, as copying the edited versions over won't allow you to revert back to the original photos. Second, iPhoto remembers which library you launched last as the default, so keep that in mind. I'd suggest that when you need to get into your hidden library, you make it a habit to open your “clean” one afterward so iPhoto will launch that by default the next time.
Of course, you could use these same steps if you're just looking to archive parts of your iPhoto Library onto an external drive or something, even if you have nothing at all to hide. But it's not as entertaining for me to imagine you doing that, so let's just go with you all being secretive, shall we?