I'm not sure of the cost/benefit of spell checking of source code, but there seem to be a couple cheap/free utilities that can help with this.
Free Python package :
Commercial shareware - $59.99 (Windows)
Another commercial shareware version - $39.99 (Windows)
Here's one man's experience checking Java source code with a "generic plain English" spell checker :
To me, it only makes sense to check the following parts of the code :
- Public documentation
- Strings and Heredocs
One nice thing is you can also check for !$#&ing profanity.
After you run these checkers against source files and setup custom dictionaries ONCE, the cost to rerun against only changes is probably pretty low. If you use something like the Python package, you can run it on the command line and automatically integrate it into the checkin/build process.
This is not a bad idea as part of a more general "continuous integration" project (http://en.wikipedia.org/wiki/Continuous_integration) but I suspect there are other projects with a higher benefit/cost profile.