[Disclosure: I work for AgileBits, the makers of 1Password. As it would be inappropriate for me to comment on the security architecture of competitors, I will address things in general, and talk specifically only about 1Password.]
Yes. Password managers create a single point of failure. You are keeping all of your eggs in one basket. I, obviously, think that a well-designed password manager is the right choice. But ultimately it is a choice that each individual needs to make for themselves.
It is extremely important to look at how that basket is protected. With 1Password you can read the details of how the data is stored. Although we make heavy use of PBKDF2, it is very important that people choose a good master password. The only confirmed case of a 1Password data breach that I've seen is when someone used the same master password as she used for her unencrypted POP3/HTTP Road Runner email. The same password was also used for her Dropbox account, which was also taken over and is how we presume the attacker obtained the 1Password data.
As for trusting the people behind a password manager, that is a trickier question. I do think that it is safe to say that anyone who is has been in the password management business for a while wouldn't risk trying to make an extra buck off of banking credentials or credit cards. Even if we were crooks at heart, that would just be bad business, as the mere suspicion of such a scheme would put the vendor out of business. Stolen credit card details sell for little more than one USD each when purchased in bulk on black markets. Banking credentials are about five times as much. The math just doesn't work for anyone whose livelihood comes from selling password management tools.
As as already been mentioned, in some schemes the data never goes to the vendor in any format. This is true of 1Password. We never see how anyone is using 1Password. However, to synchronize data across systems, we do rely on third party synching systems. So your encrypted data may be stolen from Dropbox as well as be stolen from your own computer if you use Dropbox to sync data. You should always assume that there is a non-negligible possibility that your encrypted data will be captured. This then goes back to how well your data is encrypted, which is something to look at carefully.
The other questions about trusting the suppliers of the password management system come down to trusting our competence and trusting that we haven't been coerced/bribed/"persuaded" to allow for a back door into the system. This is a lot more complicated. How do the vendors deal with security bugs as they are discovered? How much of the product's behavior and design is independently verifiable? Do the creators understand the crypto that they are using?
For systems, like 1Password, that don't have any data from users, there is very little reason for us to even be approached by government agencies (and we haven't been.) At the same time, you should assume that governments do have access to your data stored on sync systems. So again, this comes back to the question of how that data is encrypted.