TESV will be of immediate interest to everybody who bought TESIV, thats a given, and if they only wanted to reach those people they could announce it any day of the week.
But, naturally they want to make a big splash when the announce it, and grab the interest of the entire gaming market if possible. Time is on their side, as they lose nothing by holding back a few months, but stand to benefit in terms of additional pre-reveal dev time and better press saturation when they wait for a perfect lull to announce. That or they already have a date picked out and set in stone, we can't know either way until after it comes out.
If they are waiting for the perfect moment to make an announcement, they will not announce it the same week any other major game is released, or in the same week as any other schedualed announcment (like the project icarus/Bioshock Infinite thing two or three weeks back).
So don't even hope for an announcement if a blockbuster like Halo Reach or Modern Warfare: Black Ops. is being released that week, because those titles will dominate the press cycle for at least the entire week of their respective launches -- huge games have people on payroll tasked to make sure games press bloggers have new and 'exclusive' things to write posts about daily during their key marketing periods. Bethesda will wait for a lull where nothing is going on in the industry, a week like last week would be good (the 'big' release was the Dead Rising 2 pre-release mini game, and the other news was a regional price increase for Live service, no big news).
So if you're trying to outfox the marketing department at gamesas and predict the announcement, I think the first thing to look to is what else newsworthy is on the calendar. In my experience there are usually dead periods in December, but last year January and February were pretty crowded with big releases and this year might be the same. If 2011 is like 2010, there will be a great big nothing going on in March, but big releases in April. So without actually doing any homework, and looking at this year's patterns, I would say for the real optimists mid-December would be the best early window (not too late or everyone checks out for the holidays), and March 2011 would be a more reasonable bet for an uncluttered window. But, thats just off the top of my head, I don't have a release calendar in front of me at the moment.
Food for thought.
Bethesda would be able to get around all of that mess by using Game Informer. A Game Informer cover would definitely serve to bring more attention to the announcement. To get a Game Informer announcement, they'll have to announce it either at the end of the month or at the beginning of the month. Luckily for us I keep a constantly updated chart of the release dates for upcoming videogames, or at least the videogames I'm interested in, so lets see what's going on in the next few months:
September:
6th: Chime
7th: Spider-Man: Shattered Dimensions
8th: Amnesia: The Dark Descent
14th: Halo: Reach
19th: Playstation Move
21st: Etrian Odyssey 3
21st: F1 2010
23rd: Darksiders
28th: Dead Rising 2
28th: Battle vs. Chess
28th: Claptrap's New Robot Revolution
29th: Hydrophobia
30th: Final Fantasy XIV
October:
5th: Castlevania: Lords of Shadow
5th: Rock of the Dead
5th: Enslaved: Odyssey to the West
5th: Two Worlds II
5th: Nail'd
6th: Comic Jumper
8th: X Superbox
12th: Medal of Honor
12th: Arcania- Gothic 4
12th: Dragon Age: Origins Ultimate Edition
12th: Borderlands GOTY edition
19th: Fallout: New Vegas
19th: Vanquish (Xbox 360)
19th: Naruto: Ultimate Ninja Storm 2
20th: Super Meat Boy
26th: Fable III
26th: Super Scribblenauts
26th: Star Wars: The Force Unleashed 2
November:
2nd: DC Universe Online
4th: Kinect
5th: Darkstar
9th: Call of Duty: Black Ops
9th: Apache: Air Assault
16th: Need for Speed: Hot Pursuit
16th: Quantum Theory
16th: Sonic Colors
December:
7th: Tron Evolution
January:
6th: Venetica
25th: Dead Space 2
February:
9th: Portal 2
22nd: Bulletstorm
March:
8th: Dragon Age 2
22nd: Crysis 2
Of course, you have to keep in mind that those are only confirmed dates, and that there are still a lot of games coming out in "Early 2011" that will fill up January and the later months nicely. But just by looking at the current list, I'd have to agree with you that an early December announcement with a January Game Informer Cover would be ideal. But, it wouldn't really be a stretch to announce it in November either. None of those big games coming out then have the kind of fanbase who would like TES V anyway.