Hi Brad,
What are the skills required to be a pro? It all depends on the kind of music that you're playing. There are pro players with dreadful technique, no knowledge of where the notes are, crappy tone etc. who just got lucky by being in a band that got somewhere.
You mention session work. The things that are required for session work include good time, good tone, reliable equipment, an understanding of different styles of music (assuming you're doing a variety of projects - it's likely that they are going to cover a number of different styles. If you end up in a situation where you're doing lots of sessions in one style, then an intimate knowledge of that one style may well be sufficient...)
Learning scales is a step on a journey towards knowing music. Learning endless scales for a bassist would be like learning pages in a dictionary for a poet - you'd get a few choice words out of it, but it's not going to hone your craft.
So what to practice? Practice the things you're going to be required to do - stick on CDs and learn the lines as quickly as you can, trying to get the tone and the feel right as well as the notes, get used to hearing the kinds of cliches bassists use in various styles. Learn to read chord charts - this is connected with the previous practice area in that you need to know what kinds of lines to construct over the various chords. Having monster chops, being able to tap and double-thumb is unlikely to get you a session gig. It might get used on a session if you can do all that other more rudimentary things well, but it's never going to get you the gig in the first place.
So theory wise, you need to look at how basslines are constructed, how chord progressions are grouped into keys, and which notes the bassist uses a lot of the time (hint, roots and fifths come up rather a lot...)
If you're looking at doing pickup gigs as well, you'll need to be able to play Jazz standards out of a Real Book - that's a book (either legal or illegal) of jazz tunes that get called on function gigs - if you can read those, you'll be able to fill a few more night's a month. Ed Friedland's 'Building Walking Basslines' books are a great place to start for that stuff. In fact, get as many books about the fine art of bass playing as you can. John Goldsby's book on Jazz Bass is great, Chuck Sher's Improvisors Bass Method is good, The Inner Game Of Music by Barry Green is marvellous.
However, not wanting to rain on your parade, but there is VERY little studio work out there. That's not to say that you won't be one of the ones who gets the gigs, but there sadly isn't a situation where producers are sat just waiting for bassists to call up offering their availability for work so they can pay them union scale. I got offered a session recently with a multi-million selling artist, and was offered about a third of the Musician's Union minimum daily rate!! I turned it down, at least til they come back with a budget...
However, don't let that dampen your resolve to become a pro player, just remember that the area you may end up working in won't neccesarily be the one that you think is going to earn you the money...
Steve
www.stevelawson.net
Practice!? 
