How do I develop web applications for Bitcoin?
All points by Nefario are spot-on.  In addition to these, here are some Bitcoin-specific suggestions from Camp BX's experience so far:

1) Use testnet.
Testnet is an excellent resource, that unfortunately does not see as much use as it should outside the core development team.  Make sure you launch your project on testnet, and test all major subsequent releases there as well.

2) Bounties are overrated.
If you want to find bugs in your Bitcoin project, do not rely on bounties.  Perform L&amp;P testing, Unit testing, System testing on all your code and hire professional security auditors if possible.

3) Use 64-bit architecture
 (This may not apply to all projects.)  Many developers use an old 32-bit server for testing and 64-bit system as production server.  It is worth the extra investment to make sure your test server is 64-bit architecture as well.

Hope this helps.