First, there is a very high bar for what is regarded the property of the company vs your own work. Even if you created your own open source project website and pushed code on your own github account using their laptop, as long as it doesn't directly compete or use portions of proprietary code, there is zero chance it will stand in court. Non compete clauses are next to impossible to uphold unless they literally sold trade secrets or company's code directly for profit. Even if they made a near identical version, it would be a tough uphill climb if you didn't copy & paste their code directly in your own source code.
Of course, the Goldman Sachs vs open source dev was a very interesting and a rare case. I don't have more insight into that case but if somebody could that 'd be great.
But for the most part, Joel's piece is spreading FUD into innocent developer's dreams and projects, and you don't need to heed attention.
For the truly paranoid and follower of Joel's flawed legal analysis which looks at syntax than the semantics or spirit behind the written law which almost always the overwhelming use:
1) Incorporate your own company
2) Purchase laptop under your new corp
3) Do your work on there.
Disclaimer: This is not a legal advice. I'm not a lawyer. I could be wrong. Check with a real lawyer and do your own due diligence.