Assuming the check is more than a couple hundred bucks, hire an attorney for $100 to $200 to send a demand letter for the paycheck plus your attorney costs to date. (Don't let the attorney do a lot of 'research' make it clear that initially you just want a demand letter. This establishes that the expense goes up the longer they delay. Or, file a small claims action.

Any idea how I might go about finding the attorney? The websites "california employment attorney" turn off look oddly spammy. Also what kind of things would you look for in picking an attorney?

I am printing out forms for small claims now.

IMO for purposes of a demand letter on a small matter like this you could almost pick an attorney at random. Call a couple of solo lawyers who have small ads the yellow pages, explain what you want, ask for a quote for the initial demand letter. Also, instruct them to take NO further billable action after sending the letter w/o your express permission (to avoid them racking up another hour's fee to "review the reply" if your former employer sends them a reply). If they get a reply instead of a check, they should simply forward it to you. At THAT point contact your state labor board.

these guys (http://www.gcalaw.com/) are good, affordable and nice people to boot.

Talk to your state's labor board. They'll want to help you since an unpaid paycheck also means unpaid payroll taxes.

It may be easier than small claims or sending threatening letter. Something along these lines happened to me in New York state. I consulted a labor lawyer who pointed out that this was in fact illegal in the state of New York. He actually suggested I contact the state AG office. Once I explained to my former employer that they could be fined they paid up quick.

Check your state laws.

Which country are you/your employer are in? How big is your employer ? I know that in the US, it is against the law to not pay your employees for the time they worked no matter what. It is a serious offense. How much money is it ? Talk to a lawyer and take it from there. Do not do anything else on your own and keep all documentation of any communication so far where they clearly deny you your rightful paycheck on the grounds of you leaving in 2 months.

I am in California. Does employer size matter? (It was a startup, but not a tiny one)

Assuming that the other side of this story is somewhat as it has been portrayed, blogging about the issue with the potential to reveal the name of the startup might encourage them to clear up "the misunderstanding".

Heck, you might even just email someone with some equity a link to this story.

Look into California labor laws. Even though old link, I found this: http://www.labor.ca.gov/pdf/lwdalaborlaw99-03.pdf

It states "It provides that salary may be not be used as an excuse for employment termination decisions if it disparately affects older workers."