In what TLD? .com, .net, .us, or something else?
Reserved can mean numerous things depending on the TLD and the domain in question.
From what I recall, there was an instance back several years ago in which someone regged E.COM ... but didn't have it for long ... their registration info was promptly deleted out and the whois changed back with all its original dates as if the other registration had never happened.
More recently, from what I recall, numerous domains that had hyphens (-) at the end, which isn't permitted, were eventually canceled.
On the other hand, some folks have recently grabbed formerly reserved .US domains with no probs.
Again, it all depends on the TLD and the particular domain...
If you have questions regarding whether a .COM domain is reserved or not, do a whois at
http://www.internic.net/whois.html
That in some instances may answer your question ... ie. you'll find that all single character domains are actually registered already.
Some other restrictions in .COM, from my understanding are domains that start / end with "-", as well as those with "--" anywhere in them (used to be allowed, but not anymore).
Ron