My theory:

Aeris was a very mysterious character. Though she was outgoing, she held back on a lot of things at the same time.

What did Aeris know?

First of all, I think Aeris knew she was going to die. Otherwise why didn't she run away form Sephiroth? Why did she even go by herself in the first place if she planned on coming back? She knew she couldn't have defeated Sephiroth all by herself. That's why I think she knew. Also, she almost says it...
 

Aeris: '...You know... I think Hojo did something to those men in the Black Capes. But I don't know what it has to do with Sephiroth though... That's why I think we should just go after Sephiroth himself...'

  Barret: 'Yeah, me too! It's all just too damn confusin'!'

  Aeris: 'And... [pause] I'm sorry, forget it! I think I'm tired. I'm going to bed now.'

    Aeris quickly runs upstairs to her room, much to everyone's amazement.

  Cait Sith: 'What was that, all of a sudden?'
---------

I also think she planned on coming back from the dead somehow.

Cait Sith predicted: "I believe you will meet again and have a great future" .

Notice 'meet again'. That means they will be torn apart by something (Aeris' death) then meet again.

Aeris talked about the furure more than any of them. She talked about it a little bit too much even, for someone who knew she was going to die.

Aerith:
         "Just hang in there. Someday we'll look back at these hard times
and laugh."

"Next time we come we'll go on more rides."

"I'll come back when it's all over."

(there are more).

The last one, about coming back 'when it's all over'. Aeris literally tells us she is coming back, after the world is saved, and meteor and Sephiroth are destroyed. Also that is the last thing that Aeris ever says. It must mean something.

Aeris smiles when she dies. I think she knows she's going to come back.

That sentence, and "I think I can meet her there"  is also related to something Cid says on the highwind, just before they enter the Northern Crater on disc three. He is talking about a play called 'Loveless' he went and saw once on a trip to Midgar:

         "The sister of the lead asks her lover,"

(He gestures dramatically)

Cid
         "Do you really have to leave?"

(He shakes his head)

Cid
         "And the guy says,"

(He gestures again)

Cid
         "I promised. The people I love are waiting."

(He shakes his head)

Cid
         "......I don't understand. Not at all. But...... please take care
of yourself."

(He nods)

Cid
         "Of course... I'll come back to you. Even if you don't promise to
wait. I'll return knowing that you'll be here."

(He turns away and taps his foot.)

Cid
         "I remember thinking when I heard those lines,"
         "*&%! What the hell's he talkin' about? But, you know... now I'm
not so sure..."
         "I think I understand......"

It could mean absolutley nothing at all, but my interprtation for this is like this:

Cloud
         "Do you really have to leave?"

(He shakes his head)

Aeris
         "I promised. The people I love are waiting."

(He shakes his head)

Cloud
         "......I don't understand. Not at all. But...... please take care
of yourself."

(She nods)

Aeris
         "Of course... I'll come back to you. Even if you don't promise to
wait. I'll return knowing that you'll be here."

That's my interpretation. Notice I underlined.
The last thing which Cid says,          "*&%! What the hell's he talkin' about? But, you know... now I'm
not so sure..."
         "I think I understand......"

Sort of fits what Cloud says at the end: "I think I'm beginning to understand...I think I can meet her there..."

People seem to dismiss that line, but don't you see Cloud is saying he can actually go and meet
Aeris. How, we don't know.

That line is also related to what Aeris says to Cloud, "I want to meet you." Maybe she was talking about the promised land, as well as meeting the real Cloud. Perhaps that's why Square wanted you to see that date scene so much, (Aeris is the default).

Abyway, maybe now I'm looking to deep into it... but Square didn't just spend 30 million dolars on... email me aith your comments at: