Using Abalakovs:
Back the thread up with a good screw placement, test it with a bounce or two, and leave the backup in until the first person who raps has had a chance to clip a new anchor below. On the popular lines at Lake Willoughby at the end of last season, you could look 80 feet in just about any direction and find a resident v-thread. It looked like the place was grid-bolted! Resident v-threads are likely to be safe to use if they look as good as a freshly placed one. Use your own judgement and if in doubt place a new one. Remember that v-threads are cheap, your life isn't!

Beware: there has been at least one fatal accident involving reuse of an existing thread. Last year in the Canadian Rockies a climber died when he mistakenly clipped a long tail of a v-thread runner that had frozen into the ice, instead of clipping the main loop. He would likely have survived if he had backed up with a screw.

There is one potential environmental drawback to the V-Thread's popularity - they end up on the ground in the spring. So if you place them, think about touring your local ice crag in the late spring and doing a cleanup.

Can I belay off them?
Well, the jury is still out. They have been extensively and successfully field tested as rappel anchors. Mark Twight cited a University of Calgary strength test where V-Threads supported loads between 15 and 20 KN in good dry ice. This is certainly as good as most other points in a belay chain (small wired nuts can be rated as low as 4 KN!). I have not been able to review this test report though, and I have not found any other research or qualified opinion that questions or supports belaying off a thread.

Intuitively, I think I would trust a tread because studies of ice screw failure often show that the metal tubes of the screw fail before the ice into which they are placed fails. Many guides and ice climbing instructors like Jeff Lowe and Will Gadd suggest slinging icicles as pro. But until some more serious and scientific tests are conducted, I can not recommend them as points of protection. Whatever you do on lead must be a personal decision.

The Gear:
Several companies sell hooking tools for making v-threads. It is pretty darn easy to make a good one yourself, and in the next section I will describe how to do just that. But here are my reviews of the ready-mades. They all cost about $10. Besides a tool, hang a couple two foot to three foot lengths of you favorite color 9/16 supertape on a 'biner, and you are all set.

The Simond (or Braun) Abalahook is a steel cable with a swadged loop on one end (like a wire nut) and a hook on the other. It is flexible, which can be a pain to use in a deep hole. Its rubber protective cap (for the hook) can easily ride up the cable, exposing the hook to your new Gore-tex.

The Charlet Moser MultiHook is a swank solid metal job, so you can also use it to clean out ice-packed screws. It does include a useless hex nut wrench (that's the Multi in MultiHook).

Grivel's V-anchor Hook is a cheap scrap of wire without a protective cap. Not even funny! Send me $10 and I will give you my cheap scraps of old wire!

References:
"Mechanical Advantage" by John Middendorf. A fascinating history of climbing gear development, including Vitaly Abalakov's accomplishments.

"Myths, Cautions, and Techniques of Ice Screw Placement" by Chris Harmston of Black Diamond Equipment. This is the research that taught us to place ice screws pointing down rather than up. Chris is always a great read, and this is the crucial article.

"Accidents in North American Mountaineering" from the American Alpine Club. Learn from those who blow it.

Fish Products - Interesting articles on strength tests of gear, including more Chris Harmston posts on rec.climbing

"Ice Anchor Review", Joe Josephson, The Canadian Alpine Journal, 76, 1993, pp66-67

"Getting Down on a Shoestring", Murray Toft and Joe Josephson, Climbing Magazine, 124, Feb/March 1991, pp100-103

"Extreme Alpinism" Mark Twight and James Martin. Must read!

Ice links from Will Gadd