Mobile investment was the official topic for the panel of venture capitalists at today’s MobileBeat conference in Sunnyvale, but the discussion focused on a more specific, sexy topic — Apple’s iPhone 3G, and especially its App Store.

Some of the attention came just because the App Store is the hot new thing right now. It launched earlier this month, and VentureBeat writer MG Siegler described it as “sublime beyond belief.” It has captured people’s imaginations, and is bringing a lot more attention to the mobile space. With all the interest, and with a better revenue-sharing deal (Apple normally gets 30 percent, compared to 40 or 50 percent for most carrier deals, according to Menlo Ventures’ Shawn Carolan) the iPhone is pretty tempting for app developers.

But the panelists didn’t just sing hosannas to Apple. For one thing, Rich Wong of Accel Partners noted that even if the store is an improvement on the old carrier-reliant model, it’s hardly an open environment.

“Well, Apple is a walled garden,” he said. “It’s just a different form of walled garden. … Hopefully it’s a more benevolent one.”

Even Rupert Young, who handles strategic investment at AT&T (which, remember, is the only network in the U.S. that carries the iPhone), noted that the App Store isn’t a boon for all developers. Once you leave the rarefied realm of the most popular applications and get down to, say, the 50th most downloaded app, “The numbers start to get small very quickly.”

Several speakers emphasized that despite the buzz it’s getting now, focusing exclusively on the iPhone would be a mistake. Wong, for example, said he wants startups to at least “intelligently hedge their bets across multiple platforms.”

It wasn’t all about the iPhone, even if it sometimes felt that way. The VCs also offered general tips about the applications and startups that they look to invest in. One piece of advice: Don’t try to try to compete with the big players by just creating a mobile copy of their sites. Instead, look for new needs that a mobile app can address.

“If you want to allow someone to keep track of friends’ lives and message with them and you just want to do that on mobile [competing with Facebook] — don’t,” Carolan said. “It’ll just be a complete waste of time.”