Before 172 is released, maybe a little speculation about Fiona's fate is in order?
Personally, I think she's being blackmailed. If she was truly evil, Tails wouldn't be flying off in distress whenever Sonic mentions her MIA status. Plus, it would seem to negate her entire service to the Freedom Fighters, not to mention how far out of her way she's going to cover her tracks (Why would she need to tell Bean and Bark he's her boyfriend? What would they care? XD).
If I remember correctly, she was in the Knuckles comics briefly, and mentions her dislike for Sonic. Perhaps she was involved in a plot to kill/capture him back in her rogue days? Sleuth would likely be involved or have information to such a plot (it could very well have happened before he was branded a traitor, allowing him access to such info), and could use it to his advantage, given her position.
Scourge, I think, was lying about Fiona, but perhaps part of the blackmail is Fiona joining Scourge, for whatever reason. The whole thing stinks, but because of the delicateness of the situation, Tails can't say anything about it, whether out of fear of Sonic's reaction, or because it could have some catastrophic backlash if anyone takes action otherwise.
Amy ultimately finds out about it...perhaps tailing Sonic like the little fangirl she is and accidentally hearing the explanation...and flies into a tizzy.
Whatever the situation, I think Sonic/Fiona is over with 172, and Fiona will probably leave the FF's as well.
Of course, I could also be completely and utterly wrong, but hey, it only cost me two cents.
I don't know enough to truly speculate beyond the fact that I agree that Fiona isn't "evil." I don't consider Rouge to be "evil" either and that's who I've considered Fiona to be most similar to once I heard about #160-161.
The only "problem" with the Amy tailing Sonic-bit is if I'm remembering the teaser (as I'm too lazy to look it up) correctly, Amy & Julie-Su are supposed to both get in trouble. I can't see Julie-Su tailing Sonic with Amy for a fangirlish reason. ^_~
I do like the blackmail thought in general though it would have to be more than some ancient plot to do harm. That wouldn't bug anyone in the present.
I think part of the reason that Tails has been so weird about Fiona lately is not because of his crush, but because he knows something Sonic doesn't. Obviously it can't be anything life-threatening because Tails wouldn't keep that a secret, but it has to be something pretty major. In #165, Tails and Fiona were alone with Sleuth Dogg (if I recall correctly), and I reckon something must have happened "off screen" then...
Matt you maybe right, then again she might know about something that will shake the core of the freedom fighters, epspeically Bunnie and Antione (Remember their on the cover too) to the brink piont, perhaps she knew about Scrouge's switching of Patch with Antione Prime, and she's hasn't told anyone, or perhaps she knows about Dr.F's plans and is as mentioned before is being blackmailed or even being paid (Remember she once was a bounty hunter and thief) to keep Sonic and the Freedom Fighters as well as Knuckles and the Chaotix off his back.
But hey like you all that's just my unoffical guesses, looks like we'll have to wait a few more weeks to find out the true answer.
Till then, God Bless 
Yeah, I'ts most likely blackmail, although it might send Antoine and Bunnie to the brink point, it definatly won't seperate them, since they might be getting married in #174
I had a funny thought, True Red...
Rouge and Fiona were rivals when both were mercenaries. What if, considering Fiona gets dumped from the Freedom Fighters, she teams up with Rouge afterwards? XD
Hmmm, I didn't recollect Julie-Su being involved as well. I guess we'll see.
And ancient or not, if Fiona never actually buried the hatchet on her grudge with Sonic, it could leave her at a loss. And who knows...it could have happened as recently as while she was on the Freedom Fighters, perhaps sparked by his return (though she didn't seem too worked up about it, but who knows).
