Can I get the source code from an IPA?
How do I get source code or classes .m and .h files for .ipa files?
No - since the IPA file only contains the compiled project, the source code can't be obtained from this.
Certain iOS apps are open source - you can obtain the source code for those apps separately.
If the IPA file is straight from iTunes/iPhone (without any modification), the code section in the binary (as indicated by the Info.plist) is encrypted with FairPlay (Apple's proprietary DRM). If you are unsure, you can check whether the cryptid bit is set with otool (see this page for more details).