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Was wondering why I've never seen a fretless bass with the wire frets shaved flush with the fretboard, rather than pulling them, and filling in the grooves... Seems like it would in some ways be easier than removing the frets, possibly chipping the board, filling in the grooves, sanding, finishing etc. And, in case you wanted to, for some reason, go back to regular frets, it would be a simple matter (at least I think so...) to pull the frets in the traditional manner, and put in new ones.

Has anyone ever seen a fretless bass done like this, or tried it just for the heck-of-it? Would there be more cons than pros in doing this? I'm not inclined to do this to any of my basses, but it does seem like an easier way to go back to fretted if you no longer care for fretless.... what do y'all think?

I wonder if the difference would be audible. Normally fret slots (or fretlines) are made of wood fibres and glue, or strips of solid wood, and a note fingered there or sliding over it wouldn't sound any different. But metal would, I recon, even when it's not raised. It's just like those fretless basses with big inlays (for example blocks, or a tree of life like some Michael Kelly acoustics). On bare wood, a note will sound like it should: warm and woody, with a nice upright-like mwaaah. On the inlay, however, notes will sound more defined and zingy. My guess is a metal fretline, even if small, might have a similar effect.

I do think metal fretlines would look really cool
The best fretless I've ever heard and played had a full stainless steel fingerboard. It was just amazing to look at and to play, and the tone was so rich and articulate! It was a Le Fay Remington Steele. Truly amazing instrument.

"And, in case you wanted to, for some reason, go back to regular frets, it would be a simple matter (at least I think so...) to pull the frets in the traditional manner, and put in new ones."

How so? If anything, I'd think it's harder, since you've got no way to grasp the top of the fret once it's flush to the neck to either pull up and out or slide out sideways. But I guess it could depend on the fretboard material, since the approach to removal/installation varies. Certainly on a bound neck, since frets are under the binding, I'd think you're making your job more difficult. Most refrets I've seen on bound necks, for example, are placed over the binding not under.

My '72 maple bound neck J has extremely low frets, since they had worn away and my luthier had suggested filing them down rather than the major work of refretting. Eventually it will probably need a refret, but for now I've got flats on it to prolong the life of what little fret is left. The sound, however, is somewhere between a fretted and fretless, and I can approximate a full fretless tone by manipulating notes. So, in answer to your question, yes, even with low frets, the fretless sound can be found with proper technique, and I'm not sure filled wood fret slots vs. flush frets makes a noticeable difference on tone. I'm sure you guys have seen this:

http://www.youtube.com/watch?v=8CBt_FAa ... re=related

I had this discussion with a friend luthier a long time ago. The reason why you should not use steel or hard plastic fret fillers on a fretless is over the time the fretboard will get worn by the strings but not your fillers. Eventually they'll come out of the fretboard and will mess up your sound.
I also learned not to use wood putty, like Jaco did, as you have to make up the loss of ridgidity of your neck using the truss rod. If the truss rod does not have enough leeway, especially with big strings, you won't get a perfectly straight neck. A perfectly straight neck is a must on a fretless - Unlike a fretted bass where you want a bit of bow. I made this mistake when I defretted my bass but luckily the neck is straight with the truss rod maxed out.
Best thing to use are wood slices so that it worns along with the fretboard but keeps your neck rigid.

Actually, when I had my 4 string Steinberger turned into a fretless, that's exactly what the repair guy did; simply shaved the frets down as he said it made more sense than tugging them out & replacing what was already there. It still works fine & plays smoothly, no speed-bumps. I'm happy with it, it has flatwounds & mega low action, although there is some slight relief built into the neck from the start when it was fretted but of course being a graphite neck/board could be the difference that makes it so easy to play. YMMV

The difference here being a graphite board. A graphite board will not wear down like a wood board will, or be affected by humidity or temperature.