Originally Posted by: ShootaMcgavinIf i were to sit down with a couple o' guys, to you know, just jam in "Bb," would i use the Minor pentonic and its progression, or the major with its progression?[/quote]
You need more infomation. Everything depends upon two things:

1. The context (what key, what chords are in the progression).
2. Your intention.

Is it Bb major or minor? Is it a rock or country or blues tune? What kind of sound do you want to add with your guitar (happy, sad, mean, lonely)?

Originally Posted by: ShootaMcgavinBecause i've noticed that, say, A7 works well in the Aminor pentatonic box, but its not in key!

Actually some of the notes are in key. And the others provide the characteristic sound of the blues. Look at these tutorials, they will give you an explanation and some licks to play immediately:

http://www.guitartricks.com/tutorial.php?input=217
http://www.guitartricks.com/tutorial.php?input=232
http://www.guitartricks.com/tutorial.php?input=244

When you are ready for more info look here:

http://www.guitartricks.com/tutorial.php?input=296
http://www.guitartricks.com/tutorial.php?input=185
http://www.guitartricks.com/tutorial.php?input=722

[QUOTE=ShootaMcgavin]All i want to know is when someoen in a blues band says Bb or A, do they mean Major, or Minor?!

Ask them. If they don't know, then listen to the chords or watch their fingers. Most of the time they probably mean a major key with minor embellishment notes. But there are plenty of exceptions.

For a complete intro to improv look here:

http://www.guitartricks.com/tutorial.php?input=483
http://www.guitartricks.com/tutorial.php?input=491

Hope that helps. Let us know!