This is assuming they can even find out your salary accurately. And they don't know what kind of bonus you might have received.
Inflating your salary a bit is just a negotiating tactic. Considering they are sitting across from the table, trying to screw you, is it going to look that bad that you're fighting back?
There's plenty of misrepresentation going on at an interview. Of all the things to fudge, previous salary seems the least unethical. I mean after all, the potential employer isn't compelled to match or better it.
That said, I hate to lie. But man do I see it happen often.
The easiest way to assert control over this portion of the negotiation process is to refuse to focus on base salary. There's a great passage in "Being Geek" [1] that coaches you on presenting a full spectrum of compensation-related data that declaws would-be salary minimizers.
In fact, the hiring company's common insistence on focusing on base salary is a bit of a shell game, much like the classic four-square worksheet seen at a used car lot [2]. Once they can get you to agree to one number, they still have room to pressure you on some of the other parts of the deal. Luckily you can do the exact same thing to them by presenting a range of demands (base, bonus, vacation, perks) without tipping your hand and letting them know which ones mean the most to you personally. It's especially helpful to be able to weigh their benefit details against your current position without having to directly disclose your salary.
Do tech companies _ever_ negotiate those other parts of the deal? My (limited, admittedly) experience so far is that the answer is "we will under no circumstances negotiate anything but pay in a job offer." (I think I may have even been blackballed at one company recently, which I won't name but I can almost guarantee you've used their product, for asking too many questions about negotiating vacation.)
Say you get four weeks paid vacation at your current job and the new one's only offering two for new hires. Don't try to force them to give you four, just force them to toss you a bone to make up for the ten days per year of your life you're going to be giving up by coming to their company. I'm sure that's worth another 10-20% to both parties. Alternately, walk away if the four weeks really is a deal-breaker for you.
Are the people who "can't" negotiate and who blackballed you living under the same paltry vacation package you were offered? I'm assuming you didn't ask about vacation until after they made you an offer. To do otherwise is to chase off a possible opportunity before they've committed to you. Once you've got an offer, recognize that multiple parties at the hiring company are committed to seeing the deal go through, so you have more bargaining power than you did at the "we're still deciding who our top candidate is" phase.
