It varies. But definately a lot of free plugins or older plugins are especially prone to not working in 64 bit.
Professional DSP cards like TC Powercore, UAD and others are quite lacking in this regard too.
Bitbridging is hit and miss and some plugins which work in one 64 bit host don't always work in another due to different bit bridges. J-Bridge is a $20 download and probably the best one in terms of compatibility (somewhere in the 85-90% region). For now, if you are working on time critical projects, or use your DAW professionally/semi-pro/commercially, it's typically more productive/better to stick to the 32 bit version of the DAW.
http://soundcloud.com/umbriel-rising http://www.myspace.com/leedsquietmandemos Live 7.0.18 SUITE, Cubase 5.5.2], Soundforge 9, Dell XPS M1530, 2.2 Ghz C2D, 4GB, Vista Ult SP2, legit plugins a plenty, Alesis IO14.