If I understand what you need, the solution is already built in to both Aperture and Photoshop.

In Aperture, set (menu) Aperture > Preferences… > Export “External Editor File Format” to PSD (you can choose 16-bit as well, but unless you know you need it, I’d suggest that you don’t). This will allow you to save layered PSD files (as opposed to flat TIF files) within Aperture. Keep in mind you can’t interact with the layers in Aperture, but they are still there. And if you choose to “open in editor” the same file — as long as you have NOT applied any further image adjustments to it — the original layered PSD file will re-open in Aperture.

In Photoshop, if you’re using CS4, it looks like you no longer have the option (requirement) to set “maintain compatibility), unless I’m just missing it. This was an option where Photoshop would save a flattened version of the file inside the PSD document, and this allowed apps like Aperture to see the complete file. It appears that this option is gone, which I suppose means it does it automatically. If you’re using CS3, then look for that option in the preferences.

All that means is you can view the file in Aperture.

NOW… if you’re trying to create multiple versions of the same Photoshop document and have those show up in Aperture, then yes you will likely have to Save As… then import them. However what you can also do is from Aperture, if you hold down Option when you right-click (or just option-control-click) the “edit with” menu changes to “edit a copy with”. That means Aperture will duplicate the file (whatever it may be) before it opens in the editor. SO… if you have a layered PSD file in Aperture, and now you want another version of that but want to retain the current file, just “edit a copy with” and you’re off to the races.

Does that make sense, and is that what you wanted? Sounds like a good tip topic to me… I’ll get writing ;-)
-joseph

Correct. TIF is a universal, compressible, flat format, not layered. PSD is Photoshop’s native format, which retains the photoshop layers.

Read more on the TIF format here if you’re interested http://en.wikipedia.org/wiki/Tagged_Image_File_Format

As a point of comparison, PNG (not an option here, just a comparison) is a format that doesn’t retain layers, but does retain transparency. PNG is the format used by the Mac OS when you make a screenshot, so that the rounded corners, transparency and even drop shadows are maintained.

Each format has its own unique abilities, and that’s why we have so many formats out there today. PSD is basically the most flexible format—but it’s also the largest file (so you don’t use it for final delivery).

-Joseph