Despite denials from sources close to the Bahraini government, credible rumors of Saudi tanks and troops on the ground in Bahrain are widespread, as the ruling Bahraini House of Khalifa desperately reasserts control in the capital after initially ceding the central Pearl Square to tens of thousands of anti-government protesters. The House of Saud, as you may recall, has a strong interest in ensuring that the Shiite-driven unrest in Bahrain doesn't spill over to Saudi Arabia's own Shiite-manned oil fields.
In addition to the Nicholas Kristof tweet that Jesse Walker posted earlier (more here), which suggested that Saudi troops were stopping ambulances from helping protesters injured in the surprise midnight attack (and that's not the only suggestion of medics being prevented from helping), there are a few reports that Saudi tanks may have arrived on the island. One Spanish racing team owner (Bahrain was set to host the season-opening Formula 1 Grand Prix next month, something which is now very much in doubt) claimed that "there are Saudi tanks everywhere." An Iranian news organization is claiming the Saudis sent hundreds of tanks and personnel carriers in from Qatar, which it backs up with a video of armored personnel carriers rolling down a highway in Manama, though I can't confirm that those are actually from Saudi Arabia. The Guardian writes, somewhat ambiguously: "Tanks and troops from Saudi Arabia were reported to have been deployed in support of Bahraini forces."
Regardless of whether or not Saudi troops and tanks actually took part in the brutal early morning attack that dislodged the protesters from Pearl Square, the Khalifas have taken measures to prevent their own security forces from sympathizing with the mostly Shiite Bahraini protesters. For years the Sunni rulers of Bahrain have been accused of recruiting foreign riot police and naturalizing them in an effort to avoid an Egypt-like situation where low-level officers refuse orders to fire on their countrymen. As a result, few among the Bahraini security forces speak the local dialect, and some of the Pakistanis don't speak Arabic at all.
As the situation in Bahrain heats up, many commentators are casting an eye towards Saudi Arabia. Reuters reports that the Saudis "must now worry" that Bahrain's protests "may embolden" Saudi Arabia's own Shiite population. The Financial Times' Barney Jopson claims that "Bahrain has the potential to inspire political ructions in Saudi Arabia," and that "the longer protests continue in Bahrain the more [Saudi Arabia's stability] has to be questioned." Steve Soltoff at the DC-based Foundation for Defense of Democracies puts it more bluntly: "Saudi Arabia is very, very scared now."