AliasContentAccessor.cs

using System;
using System.Collections.Concurrent;
using System.Collections.Generic;
using System.Linq;
using Orchard.Alias;
using Orchard.ContentManagement;

namespace SomeNamespace.Alias {
    public class AliasContentAccessor : IAliasContentAccessor {
        ConcurrentDictionary<string, int?> _lookups;

        private readonly IContentManager _contentManager;
        private readonly IAliasService _aliasService;

        public AliasContentAccessor(IContentManager contentManager, 
                IAliasService aliasService) {
            _lookups = new ConcurrentDictionary<string, int?>();
            _contentManager = contentManager;
            _aliasService = aliasService;
        }

        public IContent GetContentItemByAlias(string alias, VersionOptions versionOptions = null) {
            versionOptions = versionOptions ?? VersionOptions.Published;
            var id = GetContentItemIdByAlias(alias, versionOptions);
            return id.HasValue ? _contentManager.Get(id.Value, versionOptions) : null;
        }

        public int? GetContentItemIdByAlias(string alias, VersionOptions versionOptions = null) {
            int? id = null;

            versionOptions = versionOptions ?? VersionOptions.Published;
            alias = (alias ?? string.Empty).Trim(new[] { '/' });

            if (!_lookups.ContainsKey(alias)) {
                var itemRoute = _aliasService.Get(alias);

                if (itemRoute != null) {
                    if (itemRoute["Id"] != null) {
                        id = itemRoute["Id"].ToString().ToNullInt();
                    }
                    else if (itemRoute["blogId"] != null) {
                        id = itemRoute["blogId"].ToString().ToNullInt();
                    }

                }

                _lookups.TryAdd(alias, id);
            }
            else {
                _lookups.TryGetValue(alias, out id);
            }

            return id;
        }
    }
}

I really like this, thanks for sharing. Just as a heads up in case... I don't know, one thing it doesn't handle is url prefixes.