I'd probably try to emphasize "left to go do a startup" on your resume, whether or not it works out, whether or not you approach it as an 80 (or even 40) hour/week thing, rather than "took time off".
But, otherwise, this is absolutely good for your resume, I think. Assuming there's no huge change in the market, you should be as employable if not more. Maybe try to stay involved in the .NET/J2EE developer community once a month or so, keep in touch with former coworkers, etc. in case you decide to go back to that instead of mobile, though.
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I'd probably try to emphasize "left to go do a startup" on your resume, whether or not it works out, whether or not you approach it as an 80 (or even 40) hour/week thing, rather than "took time off".
But, otherwise, this is absolutely good for your resume, I think. Assuming there's no huge change in the market, you should be as employable if not more. Maybe try to stay involved in the .NET/J2EE developer community once a month or so, keep in touch with former coworkers, etc. in case you decide to go back to that instead of mobile, though.