How did that work out? Well, I got a lot of "service unavailable" messages and "The request signature we calculated does not match the signature you provided" over the course of 3 days, which is probably about 3,500 DB records or so. Of those querys, the below 8 failed:

[05-Jul-2009 13:52:54] PHP Warning: SimpleDB::putAttributes(): SignatureDoesNotMatch: The request signature we calculated does not match the signature you provided. Check your AWS Secret Access Key and signing method. Consult the service documentation for details.
in sdb.php on line 636
[05-Jul-2009 13:52:57] PHP Warning: SimpleDB::putAttributes(): SignatureDoesNotMatch: The request signature we calculated does not match the signature you provided. Check your AWS Secret Access Key and signing method. Consult the service documentation for details.
in sdb.php on line 636
[05-Jul-2009 13:53:08] PHP Warning: SimpleDB::putAttributes(): SignatureDoesNotMatch: The request signature we calculated does not match the signature you provided. Check your AWS Secret Access Key and signing method. Consult the service documentation for details.
in sdb.php on line 636
[06-Jul-2009 01:07:45] PHP Warning: SimpleDB::putAttributes(): SignatureDoesNotMatch: The request signature we calculated does not match the signature you provided. Check your AWS Secret Access Key and signing method. Consult the service documentation for details.
in sdb.php on line 636
[06-Jul-2009 01:08:14] PHP Warning: SimpleDB::putAttributes(): SignatureDoesNotMatch: The request signature we calculated does not match the signature you provided. Check your AWS Secret Access Key and signing method. Consult the service documentation for details.
in sdb.php on line 636
[07-Jul-2009 04:57:45] PHP Warning: SimpleDB::putAttributes(): ServiceUnavailable: Service AmazonSimpleDB is currently unavailable. Please try again later
in sdb.php on line 636
[07-Jul-2009 13:01:48] PHP Warning: SimpleDB::putAttributes(): ServiceUnavailable: Service AmazonSimpleDB is currently unavailable. Please try again later
in sdb.php on line 636
[08-Jul-2009 06:51:28] PHP Warning: SimpleDB::putAttributes(): SignatureDoesNotMatch: The request signature we calculated does not match the signature you provided. Check your AWS Secret Access Key and signing method. Consult the service documentation for details.
in sdb.php on line 636

That's 8 too many for me. I guess SimpleDB isn't all that reliable - and even though the above is entirely fixable by writing some kind of local queue system it's a lot more work for very few benefits. Apart from scalability of course. Even then, churning up errors as above is a little alarming for such a service.

Awesome (but badly documented and exampled) PHP5 class for Amazon SimpleDB: http://sourceforge.net/projects/php-sdb/