If you want portability between Tiger and Leopard, you'll need to install your own Python framework and develop with that (there's a binary installer for 2.5.1 on www.python.org) and PyObjC (a 1.4 installer is available, or you can find 2.0 in the PyObjC repository).
To build a standalone application bundle containing a stripped-down interpreter and all dependencies, use py2app. Note that this will add approx 4MB to your .dmg/.zip distribution - basically a non-issue for broadband users and usually tolerable for dialup users.
Obviously you'll need to avoid using Leopard-only OS features, at least when your application is running on Tiger. If you want to take advantage of Leopard features when available, you could always do a version check at launch and enable/disable the relevant sections of code as needed. You might also be able to find third-party Python and/or Cocoa libraries that provide equivalent features.
If you'd like to see a typical example of a standalone PyObjC app, take a look at the ASDictionary application on the appscript downloads page (source is also available in svn). You might also want to subscribe to the PythonMac-SIG mailing list if you've not already done so.