Short answer: You don't need to have the iOS SDK on Ubuntu (nor could you anyway). build.phonegap.com takes care of that for you. There don't seem to be iOS emulators for Ubuntu but there may be some online services that you can leverage. But I believe you still need a Mac to publish to the app store.
Long answer:
You will need:
- An Apple Developer account for the certificate files
- You probably need an iOS device for testing. There don't seem to be iOS emulators for Ubuntu though there may be some good online services. (Some good threads on this: 1, 2) See Kerri's comment below highly recommending you test on a real device and not an emulator.
Here's how you would do it:
- Using your Apple Developer account, create a certificate and provisioning file in the Certificates, Identifiers & Profiles section.
- Create a .p12 file from the certificate. (You should be able to use OpenSSL but I'm not sure of the exact steps.)
- Register an account with build.phonegap.com.
- Create a signing key using your .p12 and provisioning files.
- Upload your code and build.phonegap.com will create the .ipa file for you.
- If you use an iOS device for testing, you can browse to build.phonegap.com on the device and install the app from there.
- To publish to the app store I believe you still need a Mac with Xcode. (Maybe there's a 3rd party service that does this but I couldn't find any.) An alternative to buying a Mac is to buy just the OS and either dual boot or create a virtual machine.