Hello,
The question was raised yet again, and answered properly by Deror, but I would like to add some information:
Israel is more accessible to Palestinians than many other countries. When Wikimania was held in Alexandria, Egypt, a Palestinian citizen was denied visa by the Egyptian authorities. Many Palestinians from the West Bank have permanent entry permit to Israel. Those who don't, can apply for a permit at the offices of the Palestinian Authority (the PA sends the request to Israel). In the unlikely event that this process fails, the Palestinian participant can contact the organizers and ask them to apply for a permit on his/her behalf. Palestinians from Gaza can visit Israel, but unfortunately, the process of approving their visit is time consuming due to the fact that the Hamas government does not recognize Israel. Also, the authorities in Gaza don't always approve visits to Israel or even to the West Bank (I know that because I talked with Gazan Palestinians who came to Israel)
I personally published an announcement in Arabic on the Arabic-language Wikipedia calling upon Palestinians and citizens of Arab countries to attend Wikimania 2011. In that message I gave details about the process of obtaining a visa or an entry permit.
Of course all this is irrelevant to Palestinians and Arab people who are citizens or residents of Israel (including East Jerusalem and the Golan Heights). They can visit Haifa just as a Dutch person from Amsterdam can visit Rotterdam (as Deror have already explained).
There are a lot of imaginary stories about the West Bank Barrier. I won't get into politics, because this is hardly the place for it, but the fact is that the barrier prevents ILLEGAL entry into Israel. The Green Line is a border, per Israeli and international law, and crossing it has always been subject to limitations, with the barrier or without it. Entering Israel unlawfully was unacceptable before the barrier was built just as it is today. Those who wish to enter Israel lawfully can do it today just as they always have.
Dror K