A French press is also called a cafetiere, press pot, coffee press, or coffee plunger. More information in this Wikipedia entry.
Mug & French Press Jacket
Size: to fit a mug or French press with 3.5" diameter base; pattern can be adjusted to fit different sizes.
Materials:
Yarn: 1 skein Elann Peruvian Highland Wool (worsted weight 100% Peruvian wool, 109 yds/50g). You may need more than one skein, depending on how high you want the sides to be.
Yarn subs: A smooth worsted weight yarn with good stitch definition and a little bounce; suggested subs include Cascade 220, Cascade Sierra, Plymouth Encore worsted, Brown Sheep Lamb's Pride Worsted, Southwest Trading Co. Karaoke.
Needles: US 6 (or size to get gauge), 1 set dpns or two circs [dpns or 2 circs for base, 1 circ for sides (sides are worked flat
but with built-in base, the flexibility of a circ is important)]
Gauge: 24 sts/32 rows = 4"/10cm
Notions: buttons (1 for mug, 2 for French press), tapestry needle
Basic concept:
Knit a circle from the center out large enough for base (80 sts). Work sides for 1” or up to base of handle. BO 2” or enough for width of handle. Work in desired pattern (e.g. smocked cables) for about 3” or desired height. BO all sts. Make a loop closure and sew to one open side, sew button to opposite side (two for the French press)
Variations: replace smocked pattern with plain ribbing, replace smocked pattern with stranded colorwork and bands of ribbing or moss st at top and bottom.
ABBREVIATIONS:
k = knit
beg = begin
inc = increase
kfb = knit front and back (increase)
p = purl
sl = slip (in this pattern, slip stitch as it is presented: that is, slip knit sts knitwise, purl sts purlwise)
st = stitch
sts = stitches
STITCH PATTERN
Smocking (from Barbara G. Walker's A Treasury of Knitting Patterns)
Multiple of 8 sts + 2
Rows 1 and 3 (wrong side): k2, *p2, k2*
Row 2: p2, *k2, p2*
Row 4: p2, *insert right-hand needle from front between 6th and 7th sts on left-hand needle and draw through a loop; sl this loop onto left-hand needle and knit it together with the 1st st on left hand needle; k1, p2, k2, p2*
Rows 5 and 7: Repeat rows 1 and 3
Row 6: repeat row 2
Row 8: p2, k2, p2, *draw loop from between 6th and 7th sts as before and knit it together with 1st st, then k1, p2, k2, p2*, end k2, p2
Repeat rows 1 - 8
(click on the image for a full size 6" x 2.5" version of the chart - will appear in a pop-up window)
BASE
kfb = knit into front and back of stitch
inc 1 = increase one = for this pattern, I prefer k1 into row below
With dpns or 2 circs, CO 8 sts.
Round 1: *kfb* (16 sts)
Round 2: *k2, inc 1* (24 sts)
Round 3: *inc 1, k3* (32 sts)
Round 4: *k4, inc 1* (40 sts)
Round 5: *inc 1, k5* (48 sts)
Round 6: *k6, inc 1* (56 sts)
Round 7: *inc 1, k7* (64 sts)
Round 8: *k8, inc 1* (72 sts)
Round 9: *inc 1, k9* (80 sts)
Round 10: p all sts (80 sts)
SIDES
Round 1 - 7: *k2, p2* (80 sts)
In the next round, switch from dpns or 2 circs to 1 circ
Round 8: BO 6 sts, sl1, p1, begin smocking pattern, end p2
Next 24 or so rows : work smocking pattern. On RS rows, replace first two sts of smocking pattern with sl1(purlwise), p1, and end the row with p2. On WS rows, replace first two sts of smocking pattern with sl1(knitwise), k1, and end the row with k2.
Note: work last smocked row when sides are 1" shorter than desired, then work last 8 rows in 2x2 rib. The mug used 2.5 repeats of the smocking pattern; the French press will probably use about 4.5 repeats.
BO all sts in pattern
LOOP: (for French press, make 2)
With dpns, CO 3 sts
Rows 1 - 28: work i-cord
BO
FINISHING
Exact placement and position of loop and button will vary depending on the design of the mug/French press.
Slip jacket onto mug or French press. Pin ends of loop to one of the open sides and determine placement for button on opposite open side (French press will likely need two loops and buttons). Slip jacket off and sew loop and button into place. Weave in all ends. To block, wash gently and then slip onto mug or French press to dry, turning mug or french press upside down. If you are using a French press with metal components, you may wish to wrap it in a plastic bag or plastic wrap first to keep the metal dry.

A photo tutorial for Row 4 of the Smocking Stitch pattern is available on my Flickr page.
[updated May 07, 2008]
A revised and nicely formatted PDF with instructions for knitting from the top of the sides down, as well as a chart done in Knit Visualizer, is available for $5.50 (you will need a PayPal account or a credit card). $4.50 of the purchase price will be donated to Knitters Without Borders.

The free PDF download (instructions the same as in this post - starts from the center of the base and goes up) is still available.

[this button says "buy now" but the price is set to $0.00 - some users are not able to use the "add to cart" and "view cart" buttons successfully]
In the comments for August 01, 2007, KC asked:
Mug and French Press Jacket by
MK Carroll is licensed under a
Creative Commons Attribution-Noncommercial-Share Alike 3.0 License.
Hi. Just wondering how you would adjust this pattern to fit a cup (say starbucks or the like). Perhaps in the round? This is a great pattern! I would love to use this instead of the cardboard sleeves they give you. Thanks.