Here is the situation: I have been working full-time for a customer (let's call him A) over the last month and most of this time was actually spent on projects from a single customer of them (let's call him B).
Now that I'm planning to launch a company that will be in direct competition with A, I'm wondering if I could tell B about it, give business cards and so on. How would that be perceived?
You can, but it is generally very frowned upon.
Without express written disapproval, there's nothing legally preventing you doing so. Of course, a non-compete agreement would indicate there may be repercussions were you to do anything to compete with the employer, including stealing the clients.
Stealing customers from your employer will also ensure that employer does everything they can to prevent you from working once they find out. Client "sniping" is really one clear way to gain horrible word of mouth advertising.
To hell with ethics, if you can do a better job than your "customer" and there is no paperwork such as an NDA or non-compete, then you would be doing the client a favor.
