Unfortunately telling someone that they have a "mental condition" never goes down very well.
Should you tell them?If you feel that your partner would be open-minded and willing to work on adjusting to your needs - and if you're willing to adjust to theirs, then it's worthwhile telling them. If on the other hand, you're fairly sure that your partner will simply reject the information, or that it will make them angry, then it's really not going to do any good to throw a diagnosis into the mix.
Telling your PartnerIf you do decide to talk to your partner about Asperger's syndrome, consider a more tactical approach. If you have a child with Asperger's and/or suspected Asperger's then it's easy to supply your partner with books on the subject under the guise of "divide and conquer". If you say, "we'll each read a different book on the subject and talk about what we've learned, you'll probably find that your discussions naturally lead you down the path you expected.
If on the other hand, you don't have a child who can be discussed, things are a little different.
You could try reading out a short passage from a book or from the web and saying "does this sound like you". If your partner doesn't realise that it's a diagnostic thing, they might be more open to talking about it. If all else fails, and if your partner is willing to give things a go, you can always try the RDOS Aspie test together.
The RDOS Quiz is here; http://www.rdos.net/eng/Aspie-quiz.php You don't have to logon if you don't want to, you can go straight to the test.