Porphyria's Glomschal Challenge

From LSWiki

A Challenge

Porphyria has one too many glomschals! Help her out by solving a puzzle, and taking one off her hands. The challenge: Since glomschals assist in hiding people in plain sight, the challenge is to decypher the following block of cypher text:

NFIIV TMVRVSN TIQXIR DTLHE EXALS RXZ NGVB OIMKP CG YEMGRS TYXFHTL SBOX WGPXB BAUXVO EOPTVJ

A few rules, hints, and other goodness...

• The plain text is in plain English, relatively standard grammar, with all punctuation removed and should make some sense if properly decoded.
• You may make as many guesses as you like, but be reasonable. I'll know if you're close or if you're just guessing, and reserve the right to ignore anyone who makes too many silly guesses.
• The plain text is relatively easy to decode with a pen and paper if you know the rules. It is a relatively simple cypher which can be decoded, and hopefully by somewhat-intuitive methods, not a one-time-pad, a page from the Voynich Manuscript, or any other undecypherable shenanigan. You will not need your Nazi grandmother's Enigma Machine, so put that back in her boudoir where you woke up next to it this morning.
• I will give hints when I damn well feel like it, and only on here, so don't ask for any, goddamnit, and check back here often.
• Fell free to work together on this, but there is only one glomschal, so figure out amongst yourselves who gets it. Post-award bickering will be met with slaughter and scavenger hunts.
• First in-game tell or mud mail to me with the right answer wins. In the event of a tie, I flip a coin. If the coin lands on its edge, I divine the winner via cromniomancy.

• 10/8/11 2:43 PST - I have checked the code, and I did make some mistakes, although most of it was correct. This will teach me to write this shit when I'm sleep-deprived.

FURTHER HINTS:

• Every letter is significant in decoding the plaintext. There are no 'filler characters' you're not supposed to take into account.
• It is not a ROT-based cypher.
• The cypher-to-plain text will not require rearranging the letters as an additional step (the plain text is not an anagram of some middle-step cypher). Basically, only one method was used to encrypt this.