Probably the closest, all-purpose synonyms for "newbie" are "novice" and "neophyte."
Unfortunately, I believe that for many people "novice" sounds Latinate (which it merely is) and is therefore too sophisticated for common use, and the same, of course, for neophyte. However, "newbie," in contrast, has an easier Old English, Germanic ring to it. Even though "newbie" is not a word (yet), it is based on some fairly sound principles for new-word formation and appears to me to have a fighting chance for becoming accepted as standard within 10 years. But I suspect it will remain a poor choice in formal writing for, perhaps, ever.
Ultimately, the problem has to do with the too high sounding "novice/neophyte" and the too low sounding "beginner." Beginner isn't "low speech" as such, but it is so often used in the contexts of children and other age-relative development that less confident writers are (unnecessarily) reluctant to use it.
In my mind, the best synonyms for "newbie" are: beginner, newcomer, novice, and also neophyte.
Most dictionaries will list "tyro" as a synonym on equal par with "novice," but in terms of modern, standard usage, very few people use the word "tyro" in general-purpose contexts. "Tyro" would always be a poor choice.
Beginner, despite, its perceived rhetorical shortcomings on the part of native speakers has the clear advantage for international audiences of being accurate, well-known, and clear. Therefore, whether "beginner" can always be used as a substitute for "newbie" is an excellent question. Let's see:
The FreeBSD Handbook covers a lot of information that a beginner needs.
Don't expect much, I'm just a bash/command-line beginner.
I'm a beginner at video encoding, so I'm looking for expert advice.
I'm a beginner at running Ubuntu 9.10.
It seems wrong to me, but I am a beginner with technical computer matters.
I am a beginner in the Mac world. I have a Mac mini.
I'm a total beginner with this. I've never installed Linux on anything.
It appears to me that the short answer is "yes."
I would say also that the formation "I am a newbie to X" is syntactically flawed. It appears to be derived from the standard phrasing in which "new" is used.
It seems wrong to me, but I am new to technical computer matters.
I'm totally new to this. I've never done it before.
This same type of revision can implemented in virtually all of the other sentences as well.
It covers information that people who are new to FreeBSD need.
Don't expect much, I'm still new to bash and the command line.
I'm new to video encoding, so I'm looking for expert advice.
I'm new to running Ubuntu 9.10.
I'm new to the Mac world.
In a number of cases, the "new to" edit strikes me as the cleanest and most elegant because it retains the basic idea of describing the speaker's relation to the subject (that is, "new") without marking the speaker him/herself as a particular kind of person, which is what the use of "newbie" is trying to escape in the first place by being "cute" or socially quasi-sophisticated by simply using that word token.
Just to round out the discussion, let's look at some of the alternate choices, using the full range of the various synonyms available:
The FreeBSD Handbook covers a lot of information that a novice needs.
Don't expect much, I'm just a bash/command-line beginner.
I'm a neophyte in video encoding, so I'm looking for expert advice.
I'm a novice running Ubuntu 9.10.
It seems wrong to me, but I am a newcomer to technical computer matters.
I am a neophyte in the Mac world. I have a Mac mini.
I'm a total newcomer to this. I've never installed Linux on anything.
All of these are legitimate possibilities as well.
