I'm not a lawyer, but I believe you should be fine in theory: there are no formal ICANN restrictions in place to you registering a name similar to another firms. In practice you need to be aware of trademark and "passing off" laws.
[Note that the rules for some top-level domains are different. Some registries require a company to have a trademark or a company name already established for a given domain. The .com, .org, .net and .co.uk registries are notoriously liberal, however.]
Indeed, there are plenty of examples of domain name squatters registering subtly mis-spelt variations of established domains to catch the unaware.
The main issue is one of "passing off". In some places trademark rules prevent a company trading with a similar name to another. So, for example, in the UK the famous airline easyjet.com has a policy of bringing civil suits against anyone who tries to use the "easy" prefix to their name. I'd not want to try setting up "easysoftware.com", for instance, as unless I could verify that I'd been in business for longer than Easyjet, their lawyers would be after me. Legitimate complaint or not, I don't want the distraction.
Similarly using a registered trademark in a domain such as "dell-computers-are-excellent.com" would most likely draw attention from lawyers. The example of "baa.com" is a good one.
I'd personally go by the "man-in-the-street" common-sense approach. Can your brand be confused with any other? Are you using a trademark? If so, choose a different name.