I was curious as to why they haven't bloomed, so I searched and found this below in wikipedia. Since yours have been growing for fifteen years, you may want to give them a few more years or try their suggestions before ripping them out. It sounds like they are big enough to be spectacular if they do bloom. Do you know what variety you have?

"Wisteria can be propagated via hardwood cutting, softwood cuttings, or seed. However, seeded specimens can take decades to bloom; for that reason, gardeners usually grow plants that have been started from rooted cuttings or grafted cultivars known to flower well. Another reason for failure to bloom can be excessive fertilizer (particularly nitrogen). Wisteria has nitrogen fixing capability (provided by Rhizobia bacteria in root nodules), and thus mature plants may benefit from added potassium and phosphate, but not nitrogen. Finally, wisteria can be reluctant to bloom because it has not reached maturity. Maturation may require only a few years, as in Kentucky Wisteria, or nearly twenty, as in Chinese Wisteria. Maturation can be forced by physically abusing the main trunk, root pruning, or drought stress."

Here's more on how to get them to bloom, this from an Ohio State University Extension fact sheet at
http://ohioline.osu.edu/hyg-fact/1000/1246.html
I can't parse their last sentence, although I suspect I know where the commas belong :-) you might want to give them a call or email them to be sure. Maybe their pruning hints will help prevent it spreading too much also.

"Failure to Bloom

The biggest frustration gardeners face when growing wisteria is that plants have a longer than average juvenile period and sometimes fail to bloom as expected. Start with grafted plants or those produced from cuttings rather than those grown from seed. A plant will also fail to bloom if: it does not receive full sunlight; there is excessive vegetative growth that may have been stimulated by excess nitrogen fertilizer; it is pruned heavily in winter and spring, which encourages vigorous, vegetative growth; and/or it is pruned improperly. Also, in severe winters, flower buds may be injured or killed. The following practices may help induce non-blooming vines to flower: a heavy application of superphosphate (0-20-0) in early spring (3#-5# per 100 square feet) severe pruning of new growth in late spring or early summer root pruning in late fall "

Karen Anne,
Thanks for your advice. I haven't a clue what type of wisteria it is. I bought it from one of those huge plant companies like Miller (?). It was so long ago, I can't remember, really. But it took off like crazy after I planted it. I've never fertilized it, but I do live on an old farm, so who knows where the spots of nitrogen-rich soil might be? At this point, though, the one I moved away from the house is so out of control I feel like I should write a sci-fi novel about it or report it to the authorities!

Something to watch when we're all out in the garden tonight :-) Comet goes nuts -

http://weblogs.marylandweather.com/2007/10/comet_brightens_astronomers_ab.html

The Wisteria Tree that you show in the first photo is called a standard ( a vine grafted onto the root stock of another tree ) and is better behaved than the vine. I have a Chinese Wisteria which was very mature ( 3 " caliper ) when I planted it ( A $500 tree BTW ) and it bloomed the following year.

Wisteria vines need to be pruned on a regular basis to keep them under control, otherwise they take over the world and don't bloom. Which is why you don't see that many Wisteria vines or standards -they're a lot of work to maintain.