Before starting knitting this, I’ve studied a lot of photos from the HP 3 & 4 movies with Gryffindor and Ravenclaw scarves (including the number of rows and stitches), because I wanted to make as screen-accurate scarf as possible.
(I’d be grateful if someone who owns a Gryffindor scarf from Noble Collection could check if I haven’t made any mistakes).

The original scarves are (acc. to Noble Collection) 100% lambs wool, 12” wide and 85” long (I don’t know if this includes the fringe, but judging from my scarf it doesn’t). My scarf measures about the same. It me took 2 weeks to finish it knitting several hours a day.

Update 2014-06: Lochaven of Scotland (that created the original Hogwarts school costumes) now have all House scarves available at their site here, both in original size and a smaller version. They only differ from the movie or Noble Collection scarves in that the Lochaven scarves don’t have the fringe. The weight of the original sized version is given as 300 g. Apparently, the original scarves are of a lighter yarn weight than Sports, so please keep in mind that, if you knit using my pattern, your scarf will weigh about 500 g.

Needed:

1 cone (up to 1656 yds., I had about 20 g left over) or 9 skeins of BS Nature Spun Sport weight yarn in N25 Enchanted Forest (MC - main color)
(I recommend getting a cone - it’s cheaper per yard, and you won’t have to make any knots)
(BTW, I own a Slytherin sweater from Lochaven - they supplied them for HP movies 3 and up - and the EF green shade perfectly matches the one used in the sweater);

1.5 or 2 skeins (about 280-300 yds.) of BS Nature Spun Sport in N03 Grey Heather (CC - complimentary color) (3 skeins are enough for 2 scarves)
(this color is better than 107 Silver Sage - the Ravenclaw scarves definitely have a heather grey as CC; and my Lochaven sweater too, only it’s a somewhat darker shade);

Update 2014-06: to make a lighter scarf you can use BS Nature Spun Fingering weight yarn in the same colors.

US2 size (or other 2.75-3 mm) needles, 14” single-pointed or circular
(the labels for this yarn say to use US5 size, but US2 size looks much closer to the original);

a crochet hook, 2-3 mm size - for hiding the CC threads and adding the fringe.

The entire scarf is knit in simple k1p1 rib, only switching the colors as needed.

The pattern:

cast on 116 st. (Sport weight) or 136 st. (Fingering weight) using a stretchy cast-on method (elastic cast on tutorial on Youtube);
(do not join the ends! - the scarves in the movies are not tube-shaped)

Always slip the 1st stitch in a row off, purl-wise (sl1wyif), and knit the last stitch: the first sample with a twisted edge in this great video shows the exact way I knit the first and the last stitches of my scarf. (Disclaimer: The video does not belong to me, I’ve added the link there as an example only.)

knit 1, purl 1 (simple ribbing) - repeat until the end of the needle (it must end in a knit stitch);

turn around and knit the same - sl1wyif, knit 1, purl 1.

Repeat this until you have 44 rows (always including the one on the needle).

Add the CC thread (never cut the MC thread, just carry it up between the stitches of the CC yarn), knit for 4 rows, then cut it off.

Knit in MC for 8 rows, then add the CC again and make 4 rows, then cut it off, and switch to MC.

Repeat all of the above for 13 more times, then finish off in 44 rows of MC and cast off (don’t cut off the MC thread, if you’ll be making the fringe).

The full formula for knitting a screen-accurate PoA scarf would be:
(44MC+4CC+8MC+4CC)X14+44MC
(where the numbers indicate rows)

The fringe:

(the original scarves have a machine-made fringe, so I had to improvise)

start crocheting a row of single crochet stitches (into every knit stitch only);

crochet 6 additional rows normally (add a few stitches if needed);

after you finish, add single-strand tassels to every crochet stitch (they should be 3-3,5 cm long or a bit longer than 1”).

Be careful when you wash the scarf later - the fringe felts easily (the whole scarf does).

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Nature Spun yarn colors for other Houses:

Ravenclaw - N04 Blue Knight & N03 Grey Heather (or N94 Bev’s Bear for a book-version scarf)
Gryffindor - 235 Beet Red (or 225 Brick Road) & 308 Sunburst Gold
Hufflepuff - 305 Impasse Yellow & 601 Pepper Black