So I find myself decent at improvising little melodies over a progression. And I can play some pretty gnarly and fast licks and solos that I've sat down and learned over a period of time. The thing is, I can't seem to combine the two. Whenever I try to make up some kind of fast little passage while I'm playing (eg, the stuff SRV use to do), my fingers just get tangled. Anyone have any advice for injecting speed into my on the spot solo parts?
memorise patterns and scales, arpeggios etc. it takes a while but if you keep practicing and become familiar with the fretboard improv will become a whole lot easier
as funny as it sounds, breathing has an important role in it also. I used to have the same problem but as headless said. Memorize patterns in your fretboard.

remember, is always about the SOUND = SOUL not the technique. A nice vibrato is better than playing really fast.
If you want to hear vibrato, listen to Free. Paul Kossoff is insane at it. Just practice mate, and as the guy said its not all about speed, but its nice to add it in sometimes!
Play scales over and over again until you can play them in your sleep and then start creating licks and passages outside of improv for later use. That's all what improv is - a bunch of things put together that you know sound good, whether you are thinking about it or whether it is your subconcious. That's why you can instantly recognise people like Yngwie or Friedman SRV - each improv is not a uniquely created piece of music, it's more of a compilation of what you've learned so far as a player.

Basically, learn your scales.
Learn some scales, 3nps scales will help loads. Work out some nifty sounding sequences with them, and impliment them into your playing. Just, don't get robotic, that can make the speed get annoying.