Question
Please help me to identify this plant. And tell me whether it is poisonous.  
There is familiar plant discussed on your site at
http://en.allexperts.com/q/Identifying-Unknown-Plants-722/2008/10/plant-2.htm
You called the plant Phytolacca americana.
The difference between my plant and Phytolacca is in its fruits and leaves. The plant in my garden has large leaves reaching length of 14" and width of 10". The whole plant is almost 5' tall.
The difference in fruits will be visible when you look at pictures I've found on wikipedia. Phytolacca has round berries but this plant's fruits look almost as blackberries.
Here are the links:
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http://commons.wikimedia.org/wiki/File:2009-09-27_Unknown_plant_2_%28leaf%29.JPG
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http://upload.wikimedia.org/wikipedia/commons/9/90/2009-09-27_Unknown_plant_2_%2...
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http://upload.wikimedia.org/wikipedia/commons/7/7d/2009-09-27_Unknown_plant_2_%2...
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http://upload.wikimedia.org/wikipedia/commons/6/61/2009-09-27_Unknown_plant_2_%2...
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The plant looks nice. It appeared from nowhere about 2 years ago.
For it's large green leaves, interesting fruits and overall appearance it is welcomed guest in my garden.
I will appreciate your advice.
Thank you.

Answer
I believe you have a variety of Pokeweed, Phytolacca sp., as you identified from my colleague's answer on AllExperts. There are many garden cultivars grown for their larger, showy berries in upright panicles, and with larger leaves.

It seems to me that the berries on your plant have the appearance of many pieces grown together (as in a blackberry) but that if you were to cut one open it would turn out to have only one seed.

Pokeweed is poisonous, if consumed in quantity. It will cause vomiting and can cause difficulty breathing and death. According to Peterson's Field Guide to Venomous Animals and Poisonous Plants, children have died from eating the berries of Phytolacca americana.

Your plant may be a close relative, rather than the P. americana, but I would still assume that it is poisonous and would discourage anyone from eating any part of it. Beyond that, it is not known to cause dermatitis, birds are fond of the berries, and it can be a strikingly attractive plant that many people intentionally plant in their gardens. Unless you have children under 5 who are in the habit of picking and eating blackberries I would let it grow. Should you decide to pull it barehanded, be sure to wash your hands thoroughly afterward.