good solo scale over a I IV VII rhythm?

Well... it's not I-IV-VII in A. It's I-IV-V in D (D-G-A). You'll have better luck moving that pentatonic up a couple frets - at the 7th fret its B minor pentatonic, and Bm is relative to D.

If you're looking for a single scale, that's probably it.

Thanks alot, that sounds great over it. How exactly did you figure out that it was in the key of D? I'm rather confused about determining what key a song is just from knowing the chords.

o and also, what exactly do you mean by Bm is relative to D?

relative majors and minor..

C/Am
G/Em
D/Bm
A/F#m
B/G#m

C major (C D E F G A B C) has the same notes as A minor (A B C D E F G A)

just start on VI of the major scale you get the relative minor.

if we look up..

A major (A B C# D E F# G# A) and see the 6th note is F#, then from F# to F# in that scale will be F#m (F# G# A B C# D E F#).

The chords that fall naturally into a key will all be made of notes from the same scale - they just use every other note. If you're in C, the scale is:

C-D-E-F-G-A-B-C

and the chords that fall into the key of C will be:

C major (C-E-G)
D minor (D-F-A)
E minor (E-G-B)
F major (F-A-C)
G major (G-B-D)
A minor (A-C-E)
B diminished (B-D-F)

You can see the major chords fall on I, IV, and V, and that's true of all major keys. Minor keys get a little more complicated, because there's more than one minor scale. As Alex pointed out, the minor that falls on the sixth note of a major scale is the starting point for a relative minor.

If you have three major chords, you can figure out the key. Start by assuming it's in a major key - one of the major chords will be the I chord, or tonic. In your example, you had A, D, and G.

To keep things simple for now, I won't get into the scale tones... we'll just use the letter names. You should learn the scale spellings, though, because some scales will use flat or sharp chords, like Bb or F#, and knowing the scales will help you get them right.

In your example you had A, D, and G.

If A is I, then the chords fall on I-IV-VII
if D is I, then the chords fall on I-IV-V
if G is the I, then the chords fall on I-II-V

Only D fits the I-IV-V progression, so D is the only major key that naturally contains those three chords. After you've been doing this for a while, you get to instantly recognize what key you're in from the chords in the progression.

Thanks alot, I even think I understood some of it. Is there a good book on this subject that I might get? I'm just starting to learn some theory, so I didnt totally understand everythin you said. Thanks again.

The D minor pentatonic (or D blues scale, with the flat V added) will also work over a D major accompaniment.

NoteBoat is apparently too modest to promote his own book, but since you asked, go to NoteBoat's website and check it out. :)