Is there a safe way to use bitcoin based online services? Today I've seen new service www.spend-a-bit.com.
Idea is good, however you need to send them bitcoins first and then you get your balance.

Question is: Is there any way to make online services safe to use, without worrying that you send bitcoins, but never get anything back?

Examples, which come to my mind:
1) Some website takes responsibility as dispute system. You send them first, then receive balance, then they send it to merchant. In case of arguments they check if sides kept their promises. It always takes time to figure out, which makes it less attractive for scammers.
2) Just some public ratings for services. Where everyone can report scam
Look for merchants with a good history. There will, at some point, I imagine be a listing of good and respectable merchants, ranked based on consumer reviews. Until then, just use common sense. I have yet to be scammed with Bitcoin (or USD) because I rely heavily on the reviews of other trustworthy individuals before making purchases.
Don't send more than you can afford to lose. (Bitcoin is often still referred to as an "experimental" currency for a reason.)
Even an escrow service doesn't protect you 100%. In my opinion, you're better off relying on your own intuition when placing purchases, and not buying big ticket items with Bitcoin.
Use a reputable gift card service as an intermediary. I use gyft.com to buy amazon.com gift cards, which allows me to store value in Bitcoin and also get the consumer protection offered by Amazon. (This, of course, means that I trust gyft.com, which I do -- based on a history and other consumer reviews.)