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Well, I didn't find a lot of info, so I decided to email the Dept. of Fish and Wildlife. For those interested, here's the info I got back.
The Malaysian Trumpet Snail is not prohibited in Washington and since you are importing it for an aquaria it is not regulated. This is not a species that we would want released into state waters though, so any water, plants etc that get removed from your aquaria should be disposed of into a city sewer, septic system or landfill. Information on Washington's prohibited species can be found at http://apps.leg.wa.gov/WAC/default.aspx?cite=220-12-090.
I appreciate you checking into this.
Took a few days to get the response, but here is for Washington State....