How can you steal an idea? There is little to no protection for ideas. More tangible information may have a lot of protection, but ideas are more-or-less fair game.
technical information, methods, processes, formulae, compositions, systems, techniques, inventions, machines, computer programs, research, business information, customer lists, pricing data, sources of supply, financial data, and marketing, production, or merchandising systems or plans
If your employer is working with a concept or idea and can tangibly tie it to anything like these (or any of a similar list of "artifacts") that employees had access to, then it's very dangerous for employees to start their own firms based on those concepts or ideas.
(I have no idea what's going on with Smule/Shred! Shred sounds very credible right now.)
IANAL, but here's an example:
"It would be cool to make an app that does X" <-- not likely protected
"Our marketing research shows that there is a $3B market for an app that does X" <-- likely protected, but needs some requirements to be a trade secret.
[edit]
Also, many states allow enforcement of non-compete agreements, and in some cases that may include areas the company is planning on entering at the time of your employ.
I think you should talk to a lawyer before making assessments about how unlikely it is something is to be protected. Generally: safest course is to assume it is protectable.
I talked to a lawyer before starting this company and I was very surprised by how many pitfalls I hadn't even considered.
Enforcement could be another issue, depending on evidence.
The big if is in the relevant code: "(d) Trade secret means information, including a formula, pattern, compilation, program, device, method, technique, or process, that (2) Is the subject of efforts that are reasonable under the circumstances to maintain its secrecy."
http://www.leginfo.ca.gov/cgi-bin/displaycode?file=3426-3426...