I don't like very much the Black & White conversion tools available in Aperture 3. I tried Silver Efex Pro by Nik Software and it's great, imho it's the best B&W conversion tool, but it's very expensive for me (I'm not a PRO).
Since you can easily edit your images with an external editor and save them back in Aperture, I usually use the free and open source GIMP for editing my B&W photos.
By the way, we finally have a native version of GIMP for MAC OS X, no more Apple‘s X11 needed. You can download the version you need (Lion, Mountain Lion...) from this useful page: GIMP on OS X - Downloads, open the .dmg file and drag GIMP application icon to your Application folder.
In order to set GIMP as external editor open the Aperture "Prefereces..." panel and choose the "Export" tab.
Use the "Choose..." button to select GIMP.
GIMP can read the 16 bit TIFF files, but will convert them to the 8 bit internal format, so you can choose "TIFF (8-bit)" or "PSD (8-bit)" as file format. At least not until GIMP 3.0 arrives with full support for 16 bit depth editing.
Note: if you select TIFF file format you will get this annoying warning message "wrong data type 7 for "RichTIFFIPTC"; tag ignored" every time. I was not able to avoid it so I chose PSD file format.
Now that everything is set, choose an image from your Aperture Library (I miss Kenya and the Masai Mara NP, so I chose a Zebra image I shot there last year) and from the right-click (or control-click) menu choose "Edit With Gimp..."
The editing part will not be covered here, since I'm not an expert. So when you're done editing, or when you want to save back to Aperture, the trick is to press cmd - E (or "Export" from the "File" menu) and not cmd - S (or "Save" from the "File" menu). The first time you'll need to choose the TIFF file created by Aperture (the folder will be already selected, so no worries).
You can still use cmd - S to save, but this command will save an XCF file, that is the native image format of GIMP. You can do it if you need it later.
And this is the result of my editing (basically desaturation and increase of contrast using Curves tool) that I posted yesterday on Google+:
Useful LinksGIMP on OS X Goes Native
GIMP on OS X - Downloads