Confused about loading other packages from a package

In my package, I create a new method for plot with the following signature:

setMethod("plot", signature(x="marrayNorm", y="formula"), plot.ma)

where marrayNorm is a class defined in the marray package.   After
building and installing my package, I get the following warnings when
I load my package (with library(maVis)):

Warning messages: 
1: In the method signature for function "plot", class "marrayRaw" has
no current definition in: matchSignature(signature, fdef, where)
2: In the method signature for function "plot", class "marrayRaw" has
no current definition in: matchSignature(signature, fdef, where)

(but everything appears to work ok regardless).  I have tried
require(marray) both inside and outside of the .onLoad method.

I presume this is a caused by some difference in require depending on
whether or not it is within a package or not, as simply source()ing
the code gives no errors.  What should I do?  Ignore the warnings (or
turn them off somehow?) or use a different method of loading marray?
I'm also a bit confused about what is best practice for calling
methods in other packages - use namespace:::, use require, use
library, use the depends field in DESCRIPTION, so any advice here
would also be appreciated.

Thanks for you help,

Hadley